<?xml version="1.0" encoding="US-ASCII" standalone="no"?>
<!--Generated by ThunderDome XBRL - 06:37PM UTC 2026-03-16--><link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" xlink:type="simple"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" xlink:type="simple"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" xlink:type="simple"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" xlink:type="simple"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" xlink:type="simple"/>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-22-earnings-per-common-share" xlink:href="istr-20251231.xsd#statement-note-22-earnings-per-common-share" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-22-earnings-per-common-share" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareTextBlock" xlink:label="us-gaap_EarningsPerShareTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable" xlink:label="us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_EarningsPerShareTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-21-parent-company-only-financial-statements" xlink:href="istr-20251231.xsd#statement-note-21-parent-company-only-financial-statements" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-21-parent-company-only-financial-statements" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfCondensedFinancialStatementsTable" xlink:label="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CondensedFinancialInformationOfParentCompanyOnlyDisclosureTextBlock" xlink:label="us-gaap_CondensedFinancialInformationOfParentCompanyOnlyDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_CondensedFinancialInformationOfParentCompanyOnlyDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-20-transactions-with-related-parties" xlink:href="istr-20251231.xsd#statement-note-20-transactions-with-related-parties" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-20-transactions-with-related-parties" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsDisclosureTextBlock" xlink:label="us-gaap_RelatedPartyTransactionsDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_RelatedPartyTransactionsDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-document-and-entity-information" xlink:href="istr-20251231.xsd#statement-document-and-entity-information" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-document-and-entity-information" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_AmendmentFlag" xlink:label="dei_AmendmentFlag" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_AuditorFirmId" xlink:label="dei_AuditorFirmId" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_AuditorLocation" xlink:label="dei_AuditorLocation" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_AuditorName" xlink:label="dei_AuditorName" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_CityAreaCode" xlink:label="dei_CityAreaCode" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_CurrentFiscalYearEndDate" xlink:label="dei_CurrentFiscalYearEndDate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentAnnualReport" xlink:label="dei_DocumentAnnualReport" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentFinStmtErrorCorrectionFlag" xlink:label="dei_DocumentFinStmtErrorCorrectionFlag" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentFiscalPeriodFocus" xlink:label="dei_DocumentFiscalPeriodFocus" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentFiscalYearFocus" xlink:label="dei_DocumentFiscalYearFocus" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentInformationLineItems" xlink:label="dei_DocumentInformationLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentInformationTable" xlink:label="dei_DocumentInformationTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentPeriodEndDate" xlink:label="dei_DocumentPeriodEndDate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentTransitionReport" xlink:label="dei_DocumentTransitionReport" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentType" xlink:label="dei_DocumentType" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressAddressLine1" xlink:label="dei_EntityAddressAddressLine1" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressCityOrTown" xlink:label="dei_EntityAddressCityOrTown" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressPostalZipCode" xlink:label="dei_EntityAddressPostalZipCode" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressStateOrProvince" xlink:label="dei_EntityAddressStateOrProvince" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCentralIndexKey" xlink:label="dei_EntityCentralIndexKey" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCommonStockSharesOutstanding" xlink:label="dei_EntityCommonStockSharesOutstanding" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCurrentReportingStatus" xlink:label="dei_EntityCurrentReportingStatus" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityEmergingGrowthCompany" xlink:label="dei_EntityEmergingGrowthCompany" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityFileNumber" xlink:label="dei_EntityFileNumber" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityFilerCategory" xlink:label="dei_EntityFilerCategory" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityIncorporationStateCountryCode" xlink:label="dei_EntityIncorporationStateCountryCode" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityInteractiveDataCurrent" xlink:label="dei_EntityInteractiveDataCurrent" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityPublicFloat" xlink:label="dei_EntityPublicFloat" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityRegistrantName" xlink:label="dei_EntityRegistrantName" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityShellCompany" xlink:label="dei_EntityShellCompany" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntitySmallBusiness" xlink:label="dei_EntitySmallBusiness" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityTaxIdentificationNumber" xlink:label="dei_EntityTaxIdentificationNumber" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityVoluntaryFilers" xlink:label="dei_EntityVoluntaryFilers" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityWellKnownSeasonedIssuer" xlink:label="dei_EntityWellKnownSeasonedIssuer" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_IcfrAuditorAttestationFlag" xlink:label="dei_IcfrAuditorAttestationFlag" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LocalPhoneNumber" xlink:label="dei_LocalPhoneNumber" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_Security12bTitle" xlink:label="dei_Security12bTitle" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_SecurityExchangeName" xlink:label="dei_SecurityExchangeName" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_TradingSymbol" xlink:label="dei_TradingSymbol" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="dei_DocumentInformationTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentInformationTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityCentralIndexKey" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityRegistrantName" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_AmendmentFlag" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_CurrentFiscalYearEndDate" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentFiscalPeriodFocus" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentFiscalYearFocus" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentType" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentAnnualReport" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentPeriodEndDate" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentTransitionReport" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityFileNumber" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityIncorporationStateCountryCode" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityTaxIdentificationNumber" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityAddressAddressLine1" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityAddressCityOrTown" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityAddressStateOrProvince" xlink:type="arc"/>
    <link:definitionArc order="17" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityAddressPostalZipCode" xlink:type="arc"/>
    <link:definitionArc order="18" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_CityAreaCode" xlink:type="arc"/>
    <link:definitionArc order="19" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_LocalPhoneNumber" xlink:type="arc"/>
    <link:definitionArc order="20" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_Security12bTitle" xlink:type="arc"/>
    <link:definitionArc order="21" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_TradingSymbol" xlink:type="arc"/>
    <link:definitionArc order="22" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_SecurityExchangeName" xlink:type="arc"/>
    <link:definitionArc order="23" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityWellKnownSeasonedIssuer" xlink:type="arc"/>
    <link:definitionArc order="24" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityVoluntaryFilers" xlink:type="arc"/>
    <link:definitionArc order="25" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityCurrentReportingStatus" xlink:type="arc"/>
    <link:definitionArc order="26" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityInteractiveDataCurrent" xlink:type="arc"/>
    <link:definitionArc order="27" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityFilerCategory" xlink:type="arc"/>
    <link:definitionArc order="28" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntitySmallBusiness" xlink:type="arc"/>
    <link:definitionArc order="29" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityEmergingGrowthCompany" xlink:type="arc"/>
    <link:definitionArc order="30" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_IcfrAuditorAttestationFlag" xlink:type="arc"/>
    <link:definitionArc order="31" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentFinStmtErrorCorrectionFlag" xlink:type="arc"/>
    <link:definitionArc order="32" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityShellCompany" xlink:type="arc"/>
    <link:definitionArc order="33" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityPublicFloat" xlink:type="arc"/>
    <link:definitionArc order="34" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityCommonStockSharesOutstanding" xlink:type="arc"/>
    <link:definitionArc order="35" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_AuditorName" xlink:type="arc"/>
    <link:definitionArc order="36" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_AuditorLocation" xlink:type="arc"/>
    <link:definitionArc order="37" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_AuditorFirmId" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-consolidated-balance-sheets-parentheticals" xlink:href="istr-20251231.xsd#statement-consolidated-balance-sheets-parentheticals" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-consolidated-balance-sheets-parentheticals" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:label="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="us-gaap_CommonStockSharesAuthorized" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued" xlink:label="us-gaap_CommonStockSharesIssued" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="us-gaap_CommonStockSharesOutstanding" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesFairValue" xlink:label="us-gaap_HeldToMaturitySecuritiesFairValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockDividendRatePercentage" xlink:label="us-gaap_PreferredStockDividendRatePercentage" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockLiquidationPreference" xlink:label="us-gaap_PreferredStockLiquidationPreference" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockNoParValue" xlink:label="us-gaap_PreferredStockNoParValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="us-gaap_PreferredStockSharesAuthorized" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesIssued" xlink:label="us-gaap_PreferredStockSharesIssued" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:label="us-gaap_PreferredStockSharesOutstanding" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_HeldToMaturitySecuritiesFairValue" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockNoParValue" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockSharesAuthorized" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockSharesOutstanding" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockSharesIssued" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockDividendRatePercentage" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockLiquidationPreference" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockParOrStatedValuePerShare" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesAuthorized" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesIssued" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesOutstanding" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-consolidated-statements-of-comprehensive-income-parentheticals" xlink:href="istr-20251231.xsd#statement-consolidated-statements-of-comprehensive-income-parentheticals" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-consolidated-statements-of-comprehensive-income-parentheticals" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax" xlink:label="us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax" xlink:label="us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-consolidated-statements-of-changes-in-stockholders-equity-parentheticals" xlink:href="istr-20251231.xsd#statement-consolidated-statements-of-changes-in-stockholders-equity-parentheticals" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-consolidated-statements-of-changes-in-stockholders-equity-parentheticals" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockDividendsPerShareDeclared" xlink:label="us-gaap_CommonStockDividendsPerShareDeclared" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockDividendsPerShareDeclared" xlink:label="us-gaap_PreferredStockDividendsPerShareDeclared" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockDividendsPerShareDeclared" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockDividendsPerShareDeclared" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-1-summary-of-significant-accounting-policies" xlink:href="istr-20251231.xsd#statement-note-1-summary-of-significant-accounting-policies" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-1-summary-of-significant-accounting-policies" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_CorporateStockAndMutualFundsMember" xlink:label="istr_CorporateStockAndMutualFundsMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_EquitySecuritiesHeldInCorrespondentBanksMember" xlink:label="istr_EquitySecuritiesHeldInCorrespondentBanksMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BuildingImprovementsMember" xlink:label="us-gaap_BuildingImprovementsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BuildingMember" xlink:label="us-gaap_BuildingMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComputerEquipmentMember" xlink:label="us-gaap_ComputerEquipmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CoreDepositsMember" xlink:label="us-gaap_CoreDepositsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="us-gaap_FurnitureAndFixturesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:label="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SignificantAccountingPoliciesTextBlock" xlink:label="us-gaap_SignificantAccountingPoliciesTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:to="us-gaap_FinancialInstrumentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="istr_EquitySecuritiesHeldInCorrespondentBanksMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="istr_CorporateStockAndMutualFundsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:to="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_BuildingMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_BuildingImprovementsMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:to="srt_RangeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_FurnitureAndFixturesMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_ComputerEquipmentMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:to="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_CoreDepositsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_SignificantAccountingPoliciesTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities" xlink:href="istr-20251231.xsd#statement-note-2-investment-securities" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetNotPledgedAsCollateralAndAssetPledgedAsCollateralWithoutRightMember" xlink:label="us-gaap_AssetNotPledgedAsCollateralAndAssetPledgedAsCollateralWithoutRightMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentsInDebtAndMarketableEquitySecuritiesAndCertainTradingAssetsDisclosureTextBlock" xlink:label="us-gaap_InvestmentsInDebtAndMarketableEquitySecuritiesAndCertainTradingAssetsDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgedStatusAxis" xlink:label="us-gaap_PledgedStatusAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgedStatusDomain" xlink:label="us-gaap_PledgedStatusDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgedStatusDomain" xlink:label="us-gaap_PledgedStatusDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfInvestmentsTable" xlink:label="us-gaap_ScheduleOfInvestmentsTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PledgedStatusAxis" xlink:to="us-gaap_PledgedStatusDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfInvestmentsTable" xlink:to="us-gaap_PledgedStatusAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PledgedStatusDomain" xlink:to="us-gaap_AssetNotPledgedAsCollateralAndAssetPledgedAsCollateralWithoutRightMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PledgedStatusAxis" xlink:to="us-gaap_PledgedStatusDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfInvestmentsTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfInvestmentsTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_InvestmentsInDebtAndMarketableEquitySecuritiesAndCertainTradingAssetsDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses" xlink:href="istr-20251231.xsd#statement-note-3-loans-and-allowance-for-credit-losses" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentPerformanceStatusAxis" xlink:label="us-gaap_FinancialInstrumentPerformanceStatusAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentPerformanceStatusDomain" xlink:label="us-gaap_FinancialInstrumentPerformanceStatusDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentPerformanceStatusDomain" xlink:label="us-gaap_FinancialInstrumentPerformanceStatusDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoansNotesTradeAndOtherReceivablesDisclosureTextBlock" xlink:label="us-gaap_LoansNotesTradeAndOtherReceivablesDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonperformingFinancingReceivableMember" xlink:label="us-gaap_NonperformingFinancingReceivableMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:label="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnfundedLoanCommitmentMember" xlink:label="us-gaap_UnfundedLoanCommitmentMember" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentPerformanceStatusAxis" xlink:to="us-gaap_FinancialInstrumentPerformanceStatusDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:to="us-gaap_FinancialInstrumentPerformanceStatusAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancialInstrumentPerformanceStatusDomain" xlink:to="us-gaap_NonperformingFinancingReceivableMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentPerformanceStatusAxis" xlink:to="us-gaap_FinancialInstrumentPerformanceStatusDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="us-gaap_UnfundedLoanCommitmentMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_LoansNotesTradeAndOtherReceivablesDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-4-other-real-estate-owned" xlink:href="istr-20251231.xsd#statement-note-4-other-real-estate-owned" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-4-other-real-estate-owned" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OneToFourFamilyResidentialPropertyMember" xlink:label="istr_OneToFourFamilyResidentialPropertyMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis" xlink:label="srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MortgageLoansOnRealEstateNamePropertyTypeDomain" xlink:label="srt_MortgageLoansOnRealEstateNamePropertyTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MortgageLoansOnRealEstateNamePropertyTypeDomain" xlink:label="srt_MortgageLoansOnRealEstateNamePropertyTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RealEstateOwnedTextBlock" xlink:label="us-gaap_RealEstateOwnedTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:label="us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis" xlink:to="srt_MortgageLoansOnRealEstateNamePropertyTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:to="srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_MortgageLoansOnRealEstateNamePropertyTypeDomain" xlink:to="istr_OneToFourFamilyResidentialPropertyMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis" xlink:to="srt_MortgageLoansOnRealEstateNamePropertyTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_RealEstateOwnedTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-5-bank-premises-and-equipment-" xlink:href="istr-20251231.xsd#statement-note-5-bank-premises-and-equipment-" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-5-bank-premises-and-equipment-" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_BankPremisesAndEquipmentMember" xlink:label="istr_BankPremisesAndEquipmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DiscontinuedOperationsDisposedOfByMeansOtherThanSaleMember" xlink:label="us-gaap_DiscontinuedOperationsDisposedOfByMeansOtherThanSaleMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationAxis" xlink:label="us-gaap_DisposalGroupClassificationAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain" xlink:label="us-gaap_DisposalGroupClassificationDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain" xlink:label="us-gaap_DisposalGroupClassificationDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentDisclosureTextBlock" xlink:label="us-gaap_PropertyPlantAndEquipmentDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DisposalGroupClassificationAxis" xlink:to="us-gaap_DisposalGroupClassificationDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="istr_BankPremisesAndEquipmentMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="us-gaap_DisposalGroupClassificationAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisposalGroupClassificationDomain" xlink:to="us-gaap_DiscontinuedOperationsDisposedOfByMeansOtherThanSaleMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DisposalGroupClassificationAxis" xlink:to="us-gaap_DisposalGroupClassificationDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_PropertyPlantAndEquipmentDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-6-leases" xlink:href="istr-20251231.xsd#statement-note-6-leases" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-6-leases" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeLeaseDescriptionTable" xlink:label="us-gaap_LesseeLeaseDescriptionTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeasesTextBlock" xlink:label="us-gaap_LesseeOperatingLeasesTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_LesseeLeaseDescriptionTable" xlink:to="srt_RangeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_LesseeLeaseDescriptionTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LesseeLeaseDescriptionTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_LesseeOperatingLeasesTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-7-goodwill-and-other-intangible-assets-" xlink:href="istr-20251231.xsd#statement-note-7-goodwill-and-other-intangible-assets-" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-7-goodwill-and-other-intangible-assets-" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DepreciationAndAmortizationMember" xlink:label="istr_DepreciationAndAmortizationMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CoreDepositsMember" xlink:label="us-gaap_CoreDepositsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillAndIntangibleAssetsDisclosureTextBlock" xlink:label="us-gaap_GoodwillAndIntangibleAssetsDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="us-gaap_IncomeStatementLocationAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="us-gaap_IncomeStatementLocationDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="us-gaap_IncomeStatementLocationDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:label="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="srt_RangeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_CoreDepositsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="us-gaap_IncomeStatementLocationAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeStatementLocationDomain" xlink:to="istr_DepreciationAndAmortizationMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_GoodwillAndIntangibleAssetsDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-8-deposits-" xlink:href="istr-20251231.xsd#statement-note-8-deposits-" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-8-deposits-" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetPledgedAsCollateralMember" xlink:label="us-gaap_AssetPledgedAsCollateralMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepositLiabilitiesDisclosuresTextBlock" xlink:label="us-gaap_DepositLiabilitiesDisclosuresTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepositsMember" xlink:label="us-gaap_DepositsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgedStatusAxis" xlink:label="us-gaap_PledgedStatusAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgedStatusDomain" xlink:label="us-gaap_PledgedStatusDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgedStatusDomain" xlink:label="us-gaap_PledgedStatusDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgingPurposeAxis" xlink:label="us-gaap_PledgingPurposeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgingPurposeDomain" xlink:label="us-gaap_PledgingPurposeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgingPurposeDomain" xlink:label="us-gaap_PledgingPurposeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShortTermDebtTable" xlink:label="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PledgedStatusAxis" xlink:to="us-gaap_PledgedStatusDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PledgingPurposeAxis" xlink:to="us-gaap_PledgingPurposeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="us-gaap_PledgedStatusAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PledgedStatusDomain" xlink:to="us-gaap_AssetPledgedAsCollateralMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PledgedStatusAxis" xlink:to="us-gaap_PledgedStatusDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="us-gaap_PledgingPurposeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PledgingPurposeDomain" xlink:to="us-gaap_DepositsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PledgingPurposeAxis" xlink:to="us-gaap_PledgingPurposeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_DepositLiabilitiesDisclosuresTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-9-securities-sold-under-agreements-to-repurchase-" xlink:href="istr-20251231.xsd#statement-note-9-securities-sold-under-agreements-to-repurchase-" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-9-securities-sold-under-agreements-to-repurchase-" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepurchaseAgreementsResaleAgreementsSecuritiesBorrowedAndSecuritiesLoanedDisclosureTextBlock" xlink:label="us-gaap_RepurchaseAgreementsResaleAgreementsSecuritiesBorrowedAndSecuritiesLoanedDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShortTermDebtTable" xlink:label="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_RepurchaseAgreementsResaleAgreementsSecuritiesBorrowedAndSecuritiesLoanedDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-10-subordinated-debt-securities-" xlink:href="istr-20251231.xsd#statement-note-10-subordinated-debt-securities-" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-10-subordinated-debt-securities-" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_LondonInterbankOfferRateMember" xlink:label="istr_LondonInterbankOfferRateMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="us-gaap_LongtermDebtTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="us-gaap_LongtermDebtTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="us-gaap_LongtermDebtTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShortTermDebtTable" xlink:label="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredOvernightFinancingRateSofrOvernightIndexSwapRateMember" xlink:label="us-gaap_SecuredOvernightFinancingRateSofrOvernightIndexSwapRateMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedBorrowingsDisclosureTextBlock" xlink:label="us-gaap_SubordinatedBorrowingsDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedDebtMember" xlink:label="us-gaap_SubordinatedDebtMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateAxis" xlink:label="us-gaap_VariableRateAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain" xlink:label="us-gaap_VariableRateDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain" xlink:label="us-gaap_VariableRateDomain-default" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_LongtermDebtTypeAxis" xlink:to="us-gaap_LongtermDebtTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_VariableRateAxis" xlink:to="us-gaap_VariableRateDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="us-gaap_LongtermDebtTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LongtermDebtTypeDomain" xlink:to="us-gaap_SubordinatedDebtMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_LongtermDebtTypeAxis" xlink:to="us-gaap_LongtermDebtTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="us-gaap_VariableRateAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VariableRateDomain" xlink:to="us-gaap_SecuredOvernightFinancingRateSofrOvernightIndexSwapRateMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_VariableRateAxis" xlink:to="us-gaap_VariableRateDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VariableRateDomain" xlink:to="istr_LondonInterbankOfferRateMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_SubordinatedBorrowingsDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-11-other-borrowed-funds-" xlink:href="istr-20251231.xsd#statement-note-11-other-borrowed-funds-" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-11-other-borrowed-funds-" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_BankTermFundingProgramMember" xlink:label="istr_BankTermFundingProgramMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_LoanPortfolioMember" xlink:label="istr_LoanPortfolioMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtDisclosureTextBlock" xlink:label="us-gaap_DebtDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable" xlink:label="us-gaap_DebtInstrumentTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityAxis" xlink:label="us-gaap_LineOfCreditFacilityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityLenderDomain" xlink:label="us-gaap_LineOfCreditFacilityLenderDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityLenderDomain" xlink:label="us-gaap_LineOfCreditFacilityLenderDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="us-gaap_LongtermDebtTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="us-gaap_LongtermDebtTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="us-gaap_LongtermDebtTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredDebtMember" xlink:label="us-gaap_SecuredDebtMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_LongtermDebtTypeAxis" xlink:to="us-gaap_LongtermDebtTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_LineOfCreditFacilityAxis" xlink:to="us-gaap_LineOfCreditFacilityLenderDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_FinancialInstrumentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="istr_LoanPortfolioMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_LineOfCreditFacilityAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLenderDomain" xlink:to="istr_BankTermFundingProgramMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_LineOfCreditFacilityAxis" xlink:to="us-gaap_LineOfCreditFacilityLenderDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_LongtermDebtTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LongtermDebtTypeDomain" xlink:to="us-gaap_SecuredDebtMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_LongtermDebtTypeAxis" xlink:to="us-gaap_LongtermDebtTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtInstrumentTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_DebtDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-12-derivative-financial-instruments" xlink:href="istr-20251231.xsd#statement-note-12-derivative-financial-instruments" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-12-derivative-financial-instruments" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_CustomersMember" xlink:label="istr_CustomersMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_ForwardStartingInterestRateSwapMember" xlink:label="istr_ForwardStartingInterestRateSwapMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OffsettingInterestRateSwapMember" xlink:label="istr_OffsettingInterestRateSwapMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OtherFinancialInstitutionsMember" xlink:label="istr_OtherFinancialInstitutionsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis" xlink:label="srt_CounterpartyNameAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="srt_RepurchaseAgreementCounterpartyNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="srt_RepurchaseAgreementCounterpartyNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashFlowHedgingMember" xlink:label="us-gaap_CashFlowHedgingMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosureTextBlock" xlink:label="us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis" xlink:label="us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeTable" xlink:label="us-gaap_DerivativeTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HedgingRelationshipDomain" xlink:label="us-gaap_HedgingRelationshipDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HedgingRelationshipDomain" xlink:label="us-gaap_HedgingRelationshipDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestRateSwapMember" xlink:label="us-gaap_InterestRateSwapMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_CounterpartyNameAxis" xlink:to="srt_RepurchaseAgreementCounterpartyNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis" xlink:to="us-gaap_HedgingRelationshipDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DerivativeTable" xlink:to="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="us-gaap_InterestRateSwapMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DerivativeTable" xlink:to="us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_HedgingRelationshipDomain" xlink:to="us-gaap_CashFlowHedgingMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis" xlink:to="us-gaap_HedgingRelationshipDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DerivativeTable" xlink:to="srt_CounterpartyNameAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RepurchaseAgreementCounterpartyNameDomain" xlink:to="istr_CustomersMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_CounterpartyNameAxis" xlink:to="srt_RepurchaseAgreementCounterpartyNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="istr_OffsettingInterestRateSwapMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RepurchaseAgreementCounterpartyNameDomain" xlink:to="istr_OtherFinancialInstitutionsMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="istr_ForwardStartingInterestRateSwapMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DerivativeTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-13-stockholders-equity" xlink:href="istr-20251231.xsd#statement-note-13-stockholders-equity" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-13-stockholders-equity" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember" xlink:label="us-gaap_CommonStockMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrivatePlacementMember" xlink:label="us-gaap_PrivatePlacementMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfCapitalizationEquityTable" xlink:label="us-gaap_ScheduleOfCapitalizationEquityTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesAPreferredStockMember" xlink:label="us-gaap_SeriesAPreferredStockMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="us-gaap_StatementEquityComponentsAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityNoteDisclosureTextBlock" xlink:label="us-gaap_StockholdersEquityNoteDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="us-gaap_SubsidiarySaleOfStockAxis" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfCapitalizationEquityTable" xlink:to="us-gaap_StatementClassOfStockAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_SeriesAPreferredStockMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfCapitalizationEquityTable" xlink:to="us-gaap_SubsidiarySaleOfStockAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="us-gaap_PrivatePlacementMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfCapitalizationEquityTable" xlink:to="us-gaap_StatementEquityComponentsAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_CommonStockMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfCapitalizationEquityTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfCapitalizationEquityTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_StockholdersEquityNoteDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-" xlink:href="istr-20251231.xsd#statement-note-14-stockbased-compensation-" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_A2017LongTermIncentiveCompensationPlanMember" xlink:label="istr_A2017LongTermIncentiveCompensationPlanMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_EmployeesAndDirectorsMember" xlink:label="istr_EmployeesAndDirectorsMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_EmployeesMember" xlink:label="istr_EmployeesMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_KeyPersonnelMember" xlink:label="istr_KeyPersonnelMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_NonemployeeDirectorsAndCertainOfficersMember" xlink:label="istr_NonemployeeDirectorsAndCertainOfficersMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_RestrictedStockAndRestrictedStockUnitsMember" xlink:label="istr_RestrictedStockAndRestrictedStockUnitsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_DirectorMember" xlink:label="srt_DirectorMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualAxis" xlink:label="srt_TitleOfIndividualAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="srt_TitleOfIndividualWithRelationshipToEntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock" xlink:label="us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="us-gaap_EmployeeStockOptionMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameAxis" xlink:label="us-gaap_PlanNameAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain" xlink:label="us-gaap_PlanNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain" xlink:label="us-gaap_PlanNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="us-gaap_RestrictedStockUnitsRSUMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember" xlink:label="us-gaap_ShareBasedCompensationAwardTrancheOneMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheTwoMember" xlink:label="us-gaap_ShareBasedCompensationAwardTrancheTwoMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingAxis" xlink:label="us-gaap_VestingAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain" xlink:label="us-gaap_VestingDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain" xlink:label="us-gaap_VestingDomain-default" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_VestingAxis" xlink:to="us-gaap_VestingDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_TitleOfIndividualAxis" xlink:to="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_PlanNameAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PlanNameDomain" xlink:to="istr_A2017LongTermIncentiveCompensationPlanMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="srt_TitleOfIndividualAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="istr_KeyPersonnelMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_TitleOfIndividualAxis" xlink:to="srt_TitleOfIndividualWithRelationshipToEntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_AwardTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_EmployeeStockOptionMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_VestingAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VestingDomain" xlink:to="us-gaap_ShareBasedCompensationAwardTrancheOneMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_VestingAxis" xlink:to="us-gaap_VestingDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_RestrictedStockUnitsRSUMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="istr_NonemployeeDirectorsAndCertainOfficersMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="srt_RangeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="istr_EmployeesMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="srt_DirectorMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="istr_EmployeesAndDirectorsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VestingDomain" xlink:to="us-gaap_ShareBasedCompensationAwardTrancheTwoMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="istr_RestrictedStockAndRestrictedStockUnitsMember" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-15-employee-benefits" xlink:href="istr-20251231.xsd#statement-note-15-employee-benefits" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-15-employee-benefits" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_AccruedTaxesAndOtherLiabilitiesMember" xlink:label="istr_AccruedTaxesAndOtherLiabilitiesMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_CheahaFinancialGroupMember" xlink:label="istr_CheahaFinancialGroupMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_SalariesAndEmployeeBenefitsMember" xlink:label="istr_SalariesAndEmployeeBenefitsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="us-gaap_BalanceSheetLocationAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="us-gaap_BalanceSheetLocationDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="us-gaap_BalanceSheetLocationDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="us-gaap_BusinessAcquisitionAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CompensationAndEmployeeBenefitPlansOtherThanShareBasedCompensationTextBlock" xlink:label="us-gaap_CompensationAndEmployeeBenefitPlansOtherThanShareBasedCompensationTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="us-gaap_IncomeStatementLocationAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="us-gaap_IncomeStatementLocationDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="us-gaap_IncomeStatementLocationDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:label="us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BalanceSheetLocationAxis" xlink:to="us-gaap_BalanceSheetLocationDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:to="us-gaap_BalanceSheetLocationAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BalanceSheetLocationDomain" xlink:to="istr_AccruedTaxesAndOtherLiabilitiesMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_BalanceSheetLocationAxis" xlink:to="us-gaap_BalanceSheetLocationDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:to="us-gaap_IncomeStatementLocationAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeStatementLocationDomain" xlink:to="istr_SalariesAndEmployeeBenefitsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:to="us-gaap_BusinessAcquisitionAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="istr_CheahaFinancialGroupMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_CompensationAndEmployeeBenefitPlansOtherThanShareBasedCompensationTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes" xlink:href="istr-20251231.xsd#statement-note-16-income-taxes" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DomesticCountryMember" xlink:label="us-gaap_DomesticCountryMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:label="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:label="us-gaap_IncomeTaxAuthorityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="us-gaap_IncomeTaxAuthorityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="us-gaap_IncomeTaxAuthorityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxDisclosureTextBlock" xlink:label="us-gaap_IncomeTaxDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_IncomeTaxAuthorityAxis" xlink:to="us-gaap_IncomeTaxAuthorityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:to="us-gaap_IncomeTaxAuthorityAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeTaxAuthorityDomain" xlink:to="us-gaap_DomesticCountryMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_IncomeTaxAuthorityAxis" xlink:to="us-gaap_IncomeTaxAuthorityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_IncomeTaxDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments" xlink:href="istr-20251231.xsd#statement-note-17-fair-values-of-financial-instruments" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="us-gaap_BalanceSheetLocationAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="us-gaap_BalanceSheetLocationDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="us-gaap_BalanceSheetLocationDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:label="us-gaap_FairValueByMeasurementFrequencyAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueDisclosuresTextBlock" xlink:label="us-gaap_FairValueDisclosuresTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="us-gaap_FairValueInputsLevel3Member" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasuredAtNetAssetValuePerShareMember" xlink:label="us-gaap_FairValueMeasuredAtNetAssetValuePerShareMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="us-gaap_FairValueMeasurementFrequencyDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="us-gaap_FairValueMeasurementFrequencyDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsNonrecurringMember" xlink:label="us-gaap_FairValueMeasurementsNonrecurringMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssetsMember" xlink:label="us-gaap_OtherAssetsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BalanceSheetLocationAxis" xlink:to="us-gaap_BalanceSheetLocationDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByMeasurementFrequencyAxis" xlink:to="us-gaap_FairValueMeasurementFrequencyDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:to="us-gaap_BalanceSheetLocationAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BalanceSheetLocationDomain" xlink:to="us-gaap_OtherAssetsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_BalanceSheetLocationAxis" xlink:to="us-gaap_BalanceSheetLocationDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:to="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueMeasuredAtNetAssetValuePerShareMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:to="us-gaap_FairValueByMeasurementFrequencyAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementFrequencyDomain" xlink:to="us-gaap_FairValueMeasurementsNonrecurringMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByMeasurementFrequencyAxis" xlink:to="us-gaap_FairValueMeasurementFrequencyDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel3Member" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_FairValueDisclosuresTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-18-regulatory-matters" xlink:href="istr-20251231.xsd#statement-note-18-regulatory-matters" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-18-regulatory-matters" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramTable" xlink:label="srt_ShareRepurchaseProgramTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RegulatoryCapitalRequirementsUnderBankingRegulationsTextBlock" xlink:label="us-gaap_RegulatoryCapitalRequirementsUnderBankingRegulationsTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="srt_ShareRepurchaseProgramTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="srt_ShareRepurchaseProgramTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_RegulatoryCapitalRequirementsUnderBankingRegulationsTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-19-commitments-and-contingencies" xlink:href="istr-20251231.xsd#statement-note-19-commitments-and-contingencies" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-19-commitments-and-contingencies" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommitmentsAndContingenciesDisclosureTextBlock" xlink:label="us-gaap_CommitmentsAndContingenciesDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitmentsTable" xlink:label="us-gaap_OtherCommitmentsTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_OtherCommitmentsTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherCommitmentsTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_CommitmentsAndContingenciesDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-23-subsequent-events-" xlink:href="istr-20251231.xsd#statement-note-23-subsequent-events-" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-23-subsequent-events-" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_WFBMember" xlink:label="istr_WFBMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="us-gaap_BusinessAcquisitionAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventMember" xlink:label="us-gaap_SubsequentEventMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTable" xlink:label="us-gaap_SubsequentEventTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="us-gaap_SubsequentEventTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="us-gaap_SubsequentEventTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="us-gaap_SubsequentEventTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventsTextBlock" xlink:label="us-gaap_SubsequentEventsTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_SubsequentEventTypeAxis" xlink:to="us-gaap_SubsequentEventTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_SubsequentEventTable" xlink:to="us-gaap_BusinessAcquisitionAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="istr_WFBMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_SubsequentEventTable" xlink:to="us-gaap_SubsequentEventTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SubsequentEventTypeDomain" xlink:to="us-gaap_SubsequentEventMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_SubsequentEventTypeAxis" xlink:to="us-gaap_SubsequentEventTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_SubsequentEventTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_EntityDomain" xlink:to="istr_WFBMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SubsequentEventTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_SubsequentEventsTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-significant-accounting-policies-policies" xlink:href="istr-20251231.xsd#statement-significant-accounting-policies-policies" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-significant-accounting-policies-policies" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_BankOwnedLifeInsurancePolicyTextBlock" xlink:label="istr_BankOwnedLifeInsurancePolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_NatureOfOperationsPolicyPolicyTextBlock" xlink:label="istr_NatureOfOperationsPolicyPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_PolicyTable" xlink:label="istr_PolicyTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountingPoliciesAbstract" xlink:label="us-gaap_AccountingPoliciesAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BasisOfAccountingPolicyPolicyTextBlock" xlink:label="us-gaap_BasisOfAccountingPolicyPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationsPolicy" xlink:label="us-gaap_BusinessCombinationsPolicy" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsPolicyTextBlock" xlink:label="us-gaap_CashAndCashEquivalentsPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComprehensiveIncomePolicyPolicyTextBlock" xlink:label="us-gaap_ComprehensiveIncomePolicyPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsolidationPolicyTextBlock" xlink:label="us-gaap_ConsolidationPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditLossFinancialInstrumentPolicyTextBlock" xlink:label="us-gaap_CreditLossFinancialInstrumentPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativesPolicyTextBlock" xlink:label="us-gaap_DerivativesPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerSharePolicyTextBlock" xlink:label="us-gaap_EarningsPerSharePolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinanceLoansAndLeasesReceivablePolicy" xlink:label="us-gaap_FinanceLoansAndLeasesReceivablePolicy" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillAndIntangibleAssetsPolicyTextBlock" xlink:label="us-gaap_GoodwillAndIntangibleAssetsPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPolicyTextBlock" xlink:label="us-gaap_IncomeTaxPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentPolicyTextBlock" xlink:label="us-gaap_InvestmentPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketableSecuritiesPolicy" xlink:label="us-gaap_MarketableSecuritiesPolicy" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock" xlink:label="us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OffBalanceSheetCreditExposurePolicyPolicyTextBlock" xlink:label="us-gaap_OffBalanceSheetCreditExposurePolicyPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PolicyTextBlockAbstract" xlink:label="us-gaap_PolicyTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentPolicyTextBlock" xlink:label="us-gaap_PropertyPlantAndEquipmentPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RealEstateOwnedValuationAllowancePolicy" xlink:label="us-gaap_RealEstateOwnedValuationAllowancePolicy" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepurchaseAgreementsValuationPolicy" xlink:label="us-gaap_RepurchaseAgreementsValuationPolicy" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRecognitionPolicyTextBlock" xlink:label="us-gaap_RevenueRecognitionPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingPolicyPolicyTextBlock" xlink:label="us-gaap_SegmentReportingPolicyPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationOptionAndIncentivePlansPolicy" xlink:label="us-gaap_ShareBasedCompensationOptionAndIncentivePlansPolicy" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityPolicyTextBlock" xlink:label="us-gaap_StockholdersEquityPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialAssetsTransfersOfFinancialAssetsPolicy" xlink:label="us-gaap_TransfersAndServicingOfFinancialAssetsTransfersOfFinancialAssetsPolicy" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UseOfEstimates" xlink:label="us-gaap_UseOfEstimates" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PolicyTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="istr_PolicyTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_PolicyTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PolicyTextBlockAbstract" xlink:to="us-gaap_AccountingPoliciesAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="istr_NatureOfOperationsPolicyPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_BasisOfAccountingPolicyPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_SegmentReportingPolicyPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_ConsolidationPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_UseOfEstimates" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_CashAndCashEquivalentsPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_InvestmentPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_FinanceLoansAndLeasesReceivablePolicy" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_CreditLossFinancialInstrumentPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_MarketableSecuritiesPolicy" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_PropertyPlantAndEquipmentPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_RealEstateOwnedValuationAllowancePolicy" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_GoodwillAndIntangibleAssetsPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="istr_BankOwnedLifeInsurancePolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_RepurchaseAgreementsValuationPolicy" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_ShareBasedCompensationOptionAndIncentivePlansPolicy" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_OffBalanceSheetCreditExposurePolicyPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="17" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_DerivativesPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="18" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_IncomeTaxPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="19" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_TransfersAndServicingOfFinancialAssetsTransfersOfFinancialAssetsPolicy" xlink:type="arc"/>
    <link:definitionArc order="20" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_RevenueRecognitionPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="21" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_EarningsPerSharePolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="22" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_ComprehensiveIncomePolicyPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="23" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_BusinessCombinationsPolicy" xlink:type="arc"/>
    <link:definitionArc order="24" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_StockholdersEquityPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="25" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-tables" xlink:href="istr-20251231.xsd#statement-note-2-investment-securities-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-tables" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DebtSecuritiesHeldtomaturityUnrealizedLossPositionFairValueTableTextBlock" xlink:label="istr_DebtSecuritiesHeldtomaturityUnrealizedLossPositionFairValueTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleTableTextBlock" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionFairValueTableTextBlock" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionFairValueTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesTextBlock" xlink:label="us-gaap_HeldToMaturitySecuritiesTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentsClassifiedByContractualMaturityDateTableTextBlock" xlink:label="us-gaap_InvestmentsClassifiedByContractualMaturityDateTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesReconciliationTableTextBlock" xlink:label="us-gaap_ScheduleOfAvailableForSaleSecuritiesReconciliationTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfInvestmentsTable" xlink:label="us-gaap_ScheduleOfInvestmentsTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfInvestmentsTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfInvestmentsTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfAvailableForSaleSecuritiesReconciliationTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_HeldToMaturitySecuritiesTextBlock" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionFairValueTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="istr_DebtSecuritiesHeldtomaturityUnrealizedLossPositionFairValueTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_InvestmentsClassifiedByContractualMaturityDateTableTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-tables" xlink:href="istr-20251231.xsd#statement-note-3-loans-and-allowance-for-credit-losses-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-tables" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllowanceForCreditLossesOnFinancingReceivablesTableTextBlock" xlink:label="us-gaap_AllowanceForCreditLossesOnFinancingReceivablesTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableCreditQualityIndicatorsTableTextBlock" xlink:label="us-gaap_FinancingReceivableCreditQualityIndicatorsTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PastDueFinancingReceivablesTableTextBlock" xlink:label="us-gaap_PastDueFinancingReceivablesTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTextBlock" xlink:label="us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:label="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesNonAccrualStatusTableTextBlock" xlink:label="us-gaap_ScheduleOfFinancingReceivablesNonAccrualStatusTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsTableTextBlock" xlink:label="us-gaap_ScheduleOfRelatedPartyTransactionsTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_PastDueFinancingReceivablesTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfFinancingReceivablesNonAccrualStatusTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_FinancingReceivableCreditQualityIndicatorsTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfRelatedPartyTransactionsTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_AllowanceForCreditLossesOnFinancingReceivablesTableTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-4-other-real-estate-owned-tables" xlink:href="istr-20251231.xsd#statement-note-4-other-real-estate-owned-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-4-other-real-estate-owned-tables" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherRealEstateRollForwardTableTextBlock" xlink:label="us-gaap_OtherRealEstateRollForwardTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:label="us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_OtherRealEstateRollForwardTableTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-5-bank-premises-and-equipment-tables" xlink:href="istr-20251231.xsd#statement-note-5-bank-premises-and-equipment-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-5-bank-premises-and-equipment-tables" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTextBlock" xlink:label="us-gaap_PropertyPlantAndEquipmentTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_PropertyPlantAndEquipmentTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-6-leases-tables" xlink:href="istr-20251231.xsd#statement-note-6-leases-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-6-leases-tables" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseCostTableTextBlock" xlink:label="us-gaap_LeaseCostTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeLeaseDescriptionTable" xlink:label="us-gaap_LesseeLeaseDescriptionTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseLiabilityMaturityTableTextBlock" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityMaturityTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_LesseeLeaseDescriptionTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LesseeLeaseDescriptionTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_LeaseCostTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_LesseeOperatingLeaseLiabilityMaturityTableTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-7-goodwill-and-other-intangible-assets-tables" xlink:href="istr-20251231.xsd#statement-note-7-goodwill-and-other-intangible-assets-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-7-goodwill-and-other-intangible-assets-tables" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:label="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTableTextBlock" xlink:label="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleofFiniteLivedIntangibleAssetsFutureAmortizationExpenseTableTextBlock" xlink:label="us-gaap_ScheduleofFiniteLivedIntangibleAssetsFutureAmortizationExpenseTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleofFiniteLivedIntangibleAssetsFutureAmortizationExpenseTableTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-8-deposits-tables" xlink:href="istr-20251231.xsd#statement-note-8-deposits-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-8-deposits-tables" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_ScheduledMaturitiesOfTimeDepositsTableTableTextBlock" xlink:label="istr_ScheduledMaturitiesOfTimeDepositsTableTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepositLiabilitiesTypeTableTextBlock" xlink:label="us-gaap_DepositLiabilitiesTypeTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShortTermDebtTable" xlink:label="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_DepositLiabilitiesTypeTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="istr_ScheduledMaturitiesOfTimeDepositsTableTableTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-11-other-borrowed-funds-tables" xlink:href="istr-20251231.xsd#statement-note-11-other-borrowed-funds-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-11-other-borrowed-funds-tables" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable" xlink:label="us-gaap_DebtInstrumentTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalHomeLoanBankAdvancesTableTextBlock" xlink:label="us-gaap_FederalHomeLoanBankAdvancesTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfSubordinatedBorrowingTextBlock" xlink:label="us-gaap_ScheduleOfSubordinatedBorrowingTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtInstrumentTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_FederalHomeLoanBankAdvancesTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfSubordinatedBorrowingTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-12-derivative-financial-instruments-tables" xlink:href="istr-20251231.xsd#statement-note-12-derivative-financial-instruments-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-12-derivative-financial-instruments-tables" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OffsettingAssetsAndLiabilitiesTableTextBlock" xlink:label="istr_OffsettingAssetsAndLiabilitiesTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeTable" xlink:label="us-gaap_DerivativeTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfDerivativeInstrumentsTextBlock" xlink:label="us-gaap_ScheduleOfDerivativeInstrumentsTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DerivativeTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfDerivativeInstrumentsTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="istr_OffsettingAssetsAndLiabilitiesTableTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-13-stockholders-equity-tables" xlink:href="istr-20251231.xsd#statement-note-13-stockholders-equity-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-13-stockholders-equity-tables" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAccumulatedOtherComprehensiveIncomeLossTableTextBlock" xlink:label="us-gaap_ScheduleOfAccumulatedOtherComprehensiveIncomeLossTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfCapitalizationEquityTable" xlink:label="us-gaap_ScheduleOfCapitalizationEquityTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfCapitalizationEquityTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfCapitalizationEquityTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfAccumulatedOtherComprehensiveIncomeLossTableTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-tables" xlink:href="istr-20251231.xsd#statement-note-14-stockbased-compensation-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-tables" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationRestrictedStockUnitsAwardActivityTableTextBlock" xlink:label="us-gaap_ScheduleOfShareBasedCompensationRestrictedStockUnitsAwardActivityTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock" xlink:label="us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock" xlink:label="us-gaap_ScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfShareBasedCompensationRestrictedStockUnitsAwardActivityTableTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes-tables" xlink:href="istr-20251231.xsd#statement-note-16-income-taxes-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes-tables" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:label="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfComponentsOfIncomeTaxExpenseBenefitTableTextBlock" xlink:label="us-gaap_ScheduleOfComponentsOfIncomeTaxExpenseBenefitTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfDeferredTaxAssetsAndLiabilitiesTableTextBlock" xlink:label="us-gaap_ScheduleOfDeferredTaxAssetsAndLiabilitiesTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEffectiveIncomeTaxRateReconciliationTableTextBlock" xlink:label="us-gaap_ScheduleOfEffectiveIncomeTaxRateReconciliationTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfComponentsOfIncomeTaxExpenseBenefitTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfEffectiveIncomeTaxRateReconciliationTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfDeferredTaxAssetsAndLiabilitiesTableTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments-tables" xlink:href="istr-20251231.xsd#statement-note-17-fair-values-of-financial-instruments-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments-tables" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTableTextBlock" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTextBlock" xlink:label="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByBalanceSheetGroupingTextBlock" xlink:label="us-gaap_FairValueByBalanceSheetGroupingTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisTableTextBlock" xlink:label="us-gaap_ScheduleOfFairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfFairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTextBlock" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_FairValueByBalanceSheetGroupingTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-18-regulatory-matters-tables" xlink:href="istr-20251231.xsd#statement-note-18-regulatory-matters-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-18-regulatory-matters-tables" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramTable" xlink:label="srt_ShareRepurchaseProgramTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfComplianceWithRegulatoryCapitalRequirementsUnderBankingRegulationsTextBlock" xlink:label="us-gaap_ScheduleOfComplianceWithRegulatoryCapitalRequirementsUnderBankingRegulationsTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="srt_ShareRepurchaseProgramTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="srt_ShareRepurchaseProgramTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfComplianceWithRegulatoryCapitalRequirementsUnderBankingRegulationsTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-19-commitments-and-contingencies-tables" xlink:href="istr-20251231.xsd#statement-note-19-commitments-and-contingencies-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-19-commitments-and-contingencies-tables" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OffbalancesheetAllowanceForCreditLossTableTextBlock" xlink:label="istr_OffbalancesheetAllowanceForCreditLossTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitmentsTable" xlink:label="us-gaap_OtherCommitmentsTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitmentsTableTextBlock" xlink:label="us-gaap_OtherCommitmentsTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnfundedLoanCommitmentMember" xlink:label="us-gaap_UnfundedLoanCommitmentMember" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_OtherCommitmentsTable" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="us-gaap_UnfundedLoanCommitmentMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_OtherCommitmentsTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherCommitmentsTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_OtherCommitmentsTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="istr_OffbalancesheetAllowanceForCreditLossTableTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-21-parent-company-only-financial-statements-tables" xlink:href="istr-20251231.xsd#statement-note-21-parent-company-only-financial-statements-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-21-parent-company-only-financial-statements-tables" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfCondensedBalanceSheetTableTextBlock" xlink:label="srt_ScheduleOfCondensedBalanceSheetTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfCondensedCashFlowStatementTableTextBlock" xlink:label="srt_ScheduleOfCondensedCashFlowStatementTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfCondensedFinancialStatementsTable" xlink:label="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfCondensedIncomeStatementTableTextBlock" xlink:label="srt_ScheduleOfCondensedIncomeStatementTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="srt_ScheduleOfCondensedBalanceSheetTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="srt_ScheduleOfCondensedIncomeStatementTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="srt_ScheduleOfCondensedCashFlowStatementTableTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-22-earnings-per-common-share-tables" xlink:href="istr-20251231.xsd#statement-note-22-earnings-per-common-share-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-22-earnings-per-common-share-tables" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTextBlock" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEarningsPerShareBasicAndDilutedTableTextBlock" xlink:label="us-gaap_ScheduleOfEarningsPerShareBasicAndDilutedTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable" xlink:label="us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfEarningsPerShareBasicAndDilutedTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-1-summary-of-significant-accounting-policies-details-textual" xlink:href="istr-20251231.xsd#statement-note-1-summary-of-significant-accounting-policies-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-1-summary-of-significant-accounting-policies-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_CorporateStockAndMutualFundsMember" xlink:label="istr_CorporateStockAndMutualFundsMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_EquitySecuritiesHeldInCorrespondentBanksMember" xlink:label="istr_EquitySecuritiesHeldInCorrespondentBanksMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BuildingImprovementsMember" xlink:label="us-gaap_BuildingImprovementsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BuildingMember" xlink:label="us-gaap_BuildingMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComputerEquipmentMember" xlink:label="us-gaap_ComputerEquipmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CoreDepositsMember" xlink:label="us-gaap_CoreDepositsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiCost" xlink:label="us-gaap_EquitySecuritiesFvNiCost" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife" xlink:label="us-gaap_FiniteLivedIntangibleAssetUsefulLife" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="us-gaap_FurnitureAndFixturesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillAndIntangibleAssetImpairment" xlink:label="us-gaap_GoodwillAndIntangibleAssetImpairment" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:label="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfReportableSegments" xlink:label="us-gaap_NumberOfReportableSegments" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife" xlink:label="us-gaap_PropertyPlantAndEquipmentUsefulLife" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued" xlink:label="us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:to="us-gaap_FinancialInstrumentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="istr_EquitySecuritiesHeldInCorrespondentBanksMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="istr_CorporateStockAndMutualFundsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:to="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_BuildingMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_BuildingImprovementsMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:to="srt_RangeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_FurnitureAndFixturesMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_ComputerEquipmentMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:to="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_CoreDepositsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NumberOfReportableSegments" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EquitySecuritiesFvNiCost" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentUsefulLife" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_GoodwillAndIntangibleAssetImpairment" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetUsefulLife" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-details-textual" xlink:href="istr-20251231.xsd#statement-note-2-investment-securities-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DebtSecuritiesAvailableforsaleUnrealizedLossPositionPercentOfAmortizedCostOfInvestmentSecuritiesPortfolio" xlink:label="istr_DebtSecuritiesAvailableforsaleUnrealizedLossPositionPercentOfAmortizedCostOfInvestmentSecuritiesPortfolio" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DebtSecuritiesAvailableforsaleUnrealizedLossPositionPercentOfSecuritiesCostBasis" xlink:label="istr_DebtSecuritiesAvailableforsaleUnrealizedLossPositionPercentOfSecuritiesCostBasis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetNotPledgedAsCollateralAndAssetPledgedAsCollateralWithoutRightMember" xlink:label="us-gaap_AssetNotPledgedAsCollateralAndAssetPledgedAsCollateralWithoutRightMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecurities" xlink:label="us-gaap_DebtSecurities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleAccruedInterestAfterAllowanceForCreditLoss" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleAccruedInterestAfterAllowanceForCreditLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleAccruedInterestAfterAllowanceForCreditLossStatementOfFinancialPositionExtensibleList" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleAccruedInterestAfterAllowanceForCreditLossStatementOfFinancialPositionExtensibleList" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgedStatusAxis" xlink:label="us-gaap_PledgedStatusAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgedStatusDomain" xlink:label="us-gaap_PledgedStatusDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgedStatusDomain" xlink:label="us-gaap_PledgedStatusDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfInvestmentsTable" xlink:label="us-gaap_ScheduleOfInvestmentsTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TradingSecurities" xlink:label="us-gaap_TradingSecurities" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PledgedStatusAxis" xlink:to="us-gaap_PledgedStatusDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfInvestmentsTable" xlink:to="us-gaap_PledgedStatusAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PledgedStatusDomain" xlink:to="us-gaap_AssetNotPledgedAsCollateralAndAssetPledgedAsCollateralWithoutRightMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PledgedStatusAxis" xlink:to="us-gaap_PledgedStatusDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfInvestmentsTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfInvestmentsTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TradingSecurities" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DebtSecuritiesAvailableforsaleUnrealizedLossPositionPercentOfSecuritiesCostBasis" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DebtSecuritiesAvailableforsaleUnrealizedLossPositionPercentOfAmortizedCostOfInvestmentSecuritiesPortfolio" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleAccruedInterestAfterAllowanceForCreditLoss" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecurities" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleAccruedInterestAfterAllowanceForCreditLossStatementOfFinancialPositionExtensibleList" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-details-textual" xlink:href="istr-20251231.xsd#statement-note-3-loans-and-allowance-for-credit-losses-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_LoanParticipationsAndWholeLoansSoldToAndServicedForOthersNotOnBalanceSheet" xlink:label="istr_LoanParticipationsAndWholeLoansSoldToAndServicedForOthersNotOnBalanceSheet" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_LoanParticipationsAndWholeLoansSoldToAndServicedForOthersUnpaidPrincipalBalance" xlink:label="istr_LoanParticipationsAndWholeLoansSoldToAndServicedForOthersUnpaidPrincipalBalance" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentPerformanceStatusAxis" xlink:label="us-gaap_FinancialInstrumentPerformanceStatusAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentPerformanceStatusDomain" xlink:label="us-gaap_FinancialInstrumentPerformanceStatusDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentPerformanceStatusDomain" xlink:label="us-gaap_FinancialInstrumentPerformanceStatusDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAccruedInterestAfterAllowanceForCreditLoss" xlink:label="us-gaap_FinancingReceivableAccruedInterestAfterAllowanceForCreditLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAccruedInterestAfterAllowanceForCreditLossStatementOfFinancialPositionExtensibleList" xlink:label="us-gaap_FinancingReceivableAccruedInterestAfterAllowanceForCreditLossStatementOfFinancialPositionExtensibleList" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableDeferredIncome" xlink:label="us-gaap_FinancingReceivableDeferredIncome" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRecovery" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRecovery" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoffAfterRecovery" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoffAfterRecovery" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestCreditLossExpenseReversal" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestCreditLossExpenseReversal" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount" xlink:label="us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoansAndLeasesReceivableRelatedParties" xlink:label="us-gaap_LoansAndLeasesReceivableRelatedParties" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonperformingFinancingReceivableMember" xlink:label="us-gaap_NonperformingFinancingReceivableMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProvisionForLoanLeaseAndOtherLosses" xlink:label="us-gaap_ProvisionForLoanLeaseAndOtherLosses" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:label="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnfundedLoanCommitmentMember" xlink:label="us-gaap_UnfundedLoanCommitmentMember" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentPerformanceStatusAxis" xlink:to="us-gaap_FinancialInstrumentPerformanceStatusDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:to="us-gaap_FinancialInstrumentPerformanceStatusAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancialInstrumentPerformanceStatusDomain" xlink:to="us-gaap_NonperformingFinancingReceivableMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentPerformanceStatusAxis" xlink:to="us-gaap_FinancialInstrumentPerformanceStatusDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="us-gaap_UnfundedLoanCommitmentMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableDeferredIncome" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_LoanParticipationsAndWholeLoansSoldToAndServicedForOthersNotOnBalanceSheet" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_LoanParticipationsAndWholeLoansSoldToAndServicedForOthersUnpaidPrincipalBalance" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LoansAndLeasesReceivableRelatedParties" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableAccruedInterestAfterAllowanceForCreditLoss" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ProvisionForLoanLeaseAndOtherLosses" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestCreditLossExpenseReversal" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRecovery" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoffAfterRecovery" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableAccruedInterestAfterAllowanceForCreditLossStatementOfFinancialPositionExtensibleList" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-4-other-real-estate-owned-details-textual" xlink:href="istr-20251231.xsd#statement-note-4-other-real-estate-owned-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-4-other-real-estate-owned-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OneToFourFamilyResidentialPropertyMember" xlink:label="istr_OneToFourFamilyResidentialPropertyMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OtherRealEstateOwnedWriteDowns" xlink:label="istr_OtherRealEstateOwnedWriteDowns" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis" xlink:label="srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MortgageLoansOnRealEstateNamePropertyTypeDomain" xlink:label="srt_MortgageLoansOnRealEstateNamePropertyTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MortgageLoansOnRealEstateNamePropertyTypeDomain" xlink:label="srt_MortgageLoansOnRealEstateNamePropertyTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeclosedAssets" xlink:label="us-gaap_ForeclosedAssets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MortgageLoansInProcessOfForeclosureAmount" xlink:label="us-gaap_MortgageLoansInProcessOfForeclosureAmount" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:label="us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis" xlink:to="srt_MortgageLoansOnRealEstateNamePropertyTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:to="srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_MortgageLoansOnRealEstateNamePropertyTypeDomain" xlink:to="istr_OneToFourFamilyResidentialPropertyMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis" xlink:to="srt_MortgageLoansOnRealEstateNamePropertyTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_OtherRealEstateOwnedWriteDowns" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_MortgageLoansInProcessOfForeclosureAmount" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ForeclosedAssets" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-5-bank-premises-and-equipment-details-textual" xlink:href="istr-20251231.xsd#statement-note-5-bank-premises-and-equipment-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-5-bank-premises-and-equipment-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_BankPremisesAndEquipmentMember" xlink:label="istr_BankPremisesAndEquipmentMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_NumberOfBankBranchesClosed" xlink:label="istr_NumberOfBankBranchesClosed" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationAndAmortization" xlink:label="us-gaap_DepreciationAndAmortization" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DiscontinuedOperationsDisposedOfByMeansOtherThanSaleMember" xlink:label="us-gaap_DiscontinuedOperationsDisposedOfByMeansOtherThanSaleMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationAxis" xlink:label="us-gaap_DisposalGroupClassificationAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain" xlink:label="us-gaap_DisposalGroupClassificationDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain" xlink:label="us-gaap_DisposalGroupClassificationDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnSaleOfPropertyPlantEquipment" xlink:label="us-gaap_GainLossOnSaleOfPropertyPlantEquipment" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="us-gaap_PropertyPlantAndEquipmentGross" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DisposalGroupClassificationAxis" xlink:to="us-gaap_DisposalGroupClassificationDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="istr_BankPremisesAndEquipmentMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="us-gaap_DisposalGroupClassificationAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisposalGroupClassificationDomain" xlink:to="us-gaap_DiscontinuedOperationsDisposedOfByMeansOtherThanSaleMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DisposalGroupClassificationAxis" xlink:to="us-gaap_DisposalGroupClassificationDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DepreciationAndAmortization" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_GainLossOnSaleOfPropertyPlantEquipment" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_NumberOfBankBranchesClosed" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentGross" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-6-leases-details-textual" xlink:href="istr-20251231.xsd#statement-note-6-leases-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-6-leases-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeLeaseDescriptionTable" xlink:label="us-gaap_LesseeLeaseDescriptionTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseRemainingLeaseTerm" xlink:label="us-gaap_LesseeOperatingLeaseRemainingLeaseTerm" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OccupancyNet" xlink:label="us-gaap_OccupancyNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseIncomeComprehensiveIncomeExtensibleList" xlink:label="us-gaap_OperatingLeaseIncomeComprehensiveIncomeExtensibleList" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLeaseIncome" xlink:label="us-gaap_OperatingLeaseLeaseIncome" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiability" xlink:label="us-gaap_OperatingLeaseLiability" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityStatementOfFinancialPositionExtensibleList" xlink:label="us-gaap_OperatingLeaseLiabilityStatementOfFinancialPositionExtensibleList" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAsset" xlink:label="us-gaap_OperatingLeaseRightOfUseAsset" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList" xlink:label="us-gaap_OperatingLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_LesseeLeaseDescriptionTable" xlink:to="srt_RangeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_LesseeLeaseDescriptionTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LesseeLeaseDescriptionTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeaseRightOfUseAsset" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeaseLiability" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LesseeOperatingLeaseRemainingLeaseTerm" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeaseLeaseIncome" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OccupancyNet" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeaseLiabilityStatementOfFinancialPositionExtensibleList" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeaseIncomeComprehensiveIncomeExtensibleList" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-7-goodwill-and-other-intangible-assets-details-textual" xlink:href="istr-20251231.xsd#statement-note-7-goodwill-and-other-intangible-assets-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-7-goodwill-and-other-intangible-assets-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DepreciationAndAmortizationMember" xlink:label="istr_DepreciationAndAmortizationMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfIntangibleAssets" xlink:label="us-gaap_AmortizationOfIntangibleAssets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CoreDepositsMember" xlink:label="us-gaap_CoreDepositsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife" xlink:label="us-gaap_FiniteLivedIntangibleAssetUsefulLife" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1" xlink:label="us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill" xlink:label="us-gaap_Goodwill" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillAndIntangibleAssetImpairment" xlink:label="us-gaap_GoodwillAndIntangibleAssetImpairment" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="us-gaap_IncomeStatementLocationAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="us-gaap_IncomeStatementLocationDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="us-gaap_IncomeStatementLocationDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedTrademarks" xlink:label="us-gaap_IndefiniteLivedTrademarks" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IntangibleAssetsNetIncludingGoodwill" xlink:label="us-gaap_IntangibleAssetsNetIncludingGoodwill" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:label="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="srt_RangeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_CoreDepositsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="us-gaap_IncomeStatementLocationAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeStatementLocationDomain" xlink:to="istr_DepreciationAndAmortizationMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IntangibleAssetsNetIncludingGoodwill" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_GoodwillAndIntangibleAssetImpairment" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_Goodwill" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IndefiniteLivedTrademarks" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetUsefulLife" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AmortizationOfIntangibleAssets" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-8-deposits-details-textual" xlink:href="istr-20251231.xsd#statement-note-8-deposits-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-8-deposits-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DepositsPublicFundsAmount" xlink:label="istr_DepositsPublicFundsAmount" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_TimeDeposits250000AndAbove" xlink:label="istr_TimeDeposits250000AndAbove" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetPledgedAsCollateralMember" xlink:label="us-gaap_AssetPledgedAsCollateralMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepositsMember" xlink:label="us-gaap_DepositsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalHomeLoanBankAdvances" xlink:label="us-gaap_FederalHomeLoanBankAdvances" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgedStatusAxis" xlink:label="us-gaap_PledgedStatusAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgedStatusDomain" xlink:label="us-gaap_PledgedStatusDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgedStatusDomain" xlink:label="us-gaap_PledgedStatusDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgingPurposeAxis" xlink:label="us-gaap_PledgingPurposeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgingPurposeDomain" xlink:label="us-gaap_PledgingPurposeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PledgingPurposeDomain" xlink:label="us-gaap_PledgingPurposeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDepositLiabilities" xlink:label="us-gaap_RelatedPartyDepositLiabilities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShortTermDebtTable" xlink:label="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PledgedStatusAxis" xlink:to="us-gaap_PledgedStatusDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PledgingPurposeAxis" xlink:to="us-gaap_PledgingPurposeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="us-gaap_PledgedStatusAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PledgedStatusDomain" xlink:to="us-gaap_AssetPledgedAsCollateralMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PledgedStatusAxis" xlink:to="us-gaap_PledgedStatusDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="us-gaap_PledgingPurposeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PledgingPurposeDomain" xlink:to="us-gaap_DepositsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PledgingPurposeAxis" xlink:to="us-gaap_PledgingPurposeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_TimeDeposits250000AndAbove" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DepositsPublicFundsAmount" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FederalHomeLoanBankAdvances" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_RelatedPartyDepositLiabilities" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-9-securities-sold-under-agreements-to-repurchase-details-textual" xlink:href="istr-20251231.xsd#statement-note-9-securities-sold-under-agreements-to-repurchase-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-9-securities-sold-under-agreements-to-repurchase-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_AssetsSoldUnderAgreementsToRepurchaseWeightedAverageInterestRate" xlink:label="istr_AssetsSoldUnderAgreementsToRepurchaseWeightedAverageInterestRate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate" xlink:label="us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShortTermDebtTable" xlink:label="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuritiesSoldUnderAgreementsToRepurchase" xlink:label="us-gaap_SecuritiesSoldUnderAgreementsToRepurchase" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseFairValueOfCollateral" xlink:label="us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseFairValueOfCollateral" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SecuritiesSoldUnderAgreementsToRepurchase" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseFairValueOfCollateral" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_AssetsSoldUnderAgreementsToRepurchaseWeightedAverageInterestRate" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-10-subordinated-debt-securities-details-textual" xlink:href="istr-20251231.xsd#statement-note-10-subordinated-debt-securities-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-10-subordinated-debt-securities-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_LondonInterbankOfferRateMember" xlink:label="istr_LondonInterbankOfferRateMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" xlink:label="us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount" xlink:label="us-gaap_DebtInstrumentFaceAmount" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRepurchasedFaceAmount" xlink:label="us-gaap_DebtInstrumentRepurchasedFaceAmount" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredFinanceCostsNet" xlink:label="us-gaap_DeferredFinanceCostsNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="us-gaap_LongtermDebtTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="us-gaap_LongtermDebtTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="us-gaap_LongtermDebtTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShortTermDebtTable" xlink:label="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredOvernightFinancingRateSofrOvernightIndexSwapRateMember" xlink:label="us-gaap_SecuredOvernightFinancingRateSofrOvernightIndexSwapRateMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedDebt" xlink:label="us-gaap_SubordinatedDebt" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedDebtMember" xlink:label="us-gaap_SubordinatedDebtMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateAxis" xlink:label="us-gaap_VariableRateAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain" xlink:label="us-gaap_VariableRateDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain" xlink:label="us-gaap_VariableRateDomain-default" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_LongtermDebtTypeAxis" xlink:to="us-gaap_LongtermDebtTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_VariableRateAxis" xlink:to="us-gaap_VariableRateDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="us-gaap_LongtermDebtTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LongtermDebtTypeDomain" xlink:to="us-gaap_SubordinatedDebtMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_LongtermDebtTypeAxis" xlink:to="us-gaap_LongtermDebtTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="us-gaap_VariableRateAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VariableRateDomain" xlink:to="us-gaap_SecuredOvernightFinancingRateSofrOvernightIndexSwapRateMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_VariableRateAxis" xlink:to="us-gaap_VariableRateDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VariableRateDomain" xlink:to="istr_LondonInterbankOfferRateMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtInstrumentFaceAmount" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtInstrumentRepurchasedFaceAmount" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SubordinatedDebt" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredFinanceCostsNet" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-11-other-borrowed-funds-details-textual" xlink:href="istr-20251231.xsd#statement-note-11-other-borrowed-funds-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-11-other-borrowed-funds-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_BankTermFundingProgramMember" xlink:label="istr_BankTermFundingProgramMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_LoanPortfolioMember" xlink:label="istr_LoanPortfolioMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable" xlink:label="us-gaap_DebtInstrumentTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresAmountOfAvailableUnusedFunds" xlink:label="us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresAmountOfAvailableUnusedFunds" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1" xlink:label="us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust" xlink:label="us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCredit" xlink:label="us-gaap_LineOfCredit" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityAxis" xlink:label="us-gaap_LineOfCreditFacilityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity" xlink:label="us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityLenderDomain" xlink:label="us-gaap_LineOfCreditFacilityLenderDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityLenderDomain" xlink:label="us-gaap_LineOfCreditFacilityLenderDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="us-gaap_LongtermDebtTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="us-gaap_LongtermDebtTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="us-gaap_LongtermDebtTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherShortTermBorrowings" xlink:label="us-gaap_OtherShortTermBorrowings" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredDebtMember" xlink:label="us-gaap_SecuredDebtMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_LongtermDebtTypeAxis" xlink:to="us-gaap_LongtermDebtTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_LineOfCreditFacilityAxis" xlink:to="us-gaap_LineOfCreditFacilityLenderDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_FinancialInstrumentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="istr_LoanPortfolioMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_LineOfCreditFacilityAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLenderDomain" xlink:to="istr_BankTermFundingProgramMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_LineOfCreditFacilityAxis" xlink:to="us-gaap_LineOfCreditFacilityLenderDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_LongtermDebtTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LongtermDebtTypeDomain" xlink:to="us-gaap_SecuredDebtMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_LongtermDebtTypeAxis" xlink:to="us-gaap_LongtermDebtTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtInstrumentTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresAmountOfAvailableUnusedFunds" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherShortTermBorrowings" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LineOfCredit" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-12-derivative-financial-instruments-details-textual" xlink:href="istr-20251231.xsd#statement-note-12-derivative-financial-instruments-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-12-derivative-financial-instruments-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_CustomersMember" xlink:label="istr_CustomersMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DerivativeCollateralPost" xlink:label="istr_DerivativeCollateralPost" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_ForwardStartingInterestRateSwapMember" xlink:label="istr_ForwardStartingInterestRateSwapMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OffsettingInterestRateSwapMember" xlink:label="istr_OffsettingInterestRateSwapMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OtherFinancialInstitutionsMember" xlink:label="istr_OtherFinancialInstitutionsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis" xlink:label="srt_CounterpartyNameAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="srt_RepurchaseAgreementCounterpartyNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="srt_RepurchaseAgreementCounterpartyNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashFlowHedgingMember" xlink:label="us-gaap_CashFlowHedgingMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeGainLossOnDerivativeNet" xlink:label="us-gaap_DerivativeGainLossOnDerivativeNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis" xlink:label="us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeLiabilityFairValueGrossAsset" xlink:label="us-gaap_DerivativeLiabilityFairValueGrossAsset" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeNotionalAmount" xlink:label="us-gaap_DerivativeNotionalAmount" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeTable" xlink:label="us-gaap_DerivativeTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HedgingRelationshipDomain" xlink:label="us-gaap_HedgingRelationshipDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HedgingRelationshipDomain" xlink:label="us-gaap_HedgingRelationshipDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestRateSwapMember" xlink:label="us-gaap_InterestRateSwapMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_CounterpartyNameAxis" xlink:to="srt_RepurchaseAgreementCounterpartyNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis" xlink:to="us-gaap_HedgingRelationshipDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DerivativeTable" xlink:to="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="us-gaap_InterestRateSwapMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DerivativeTable" xlink:to="us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_HedgingRelationshipDomain" xlink:to="us-gaap_CashFlowHedgingMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis" xlink:to="us-gaap_HedgingRelationshipDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DerivativeTable" xlink:to="srt_CounterpartyNameAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RepurchaseAgreementCounterpartyNameDomain" xlink:to="istr_CustomersMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_CounterpartyNameAxis" xlink:to="srt_RepurchaseAgreementCounterpartyNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="istr_OffsettingInterestRateSwapMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RepurchaseAgreementCounterpartyNameDomain" xlink:to="istr_OtherFinancialInstitutionsMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="istr_ForwardStartingInterestRateSwapMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DerivativeTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeLiabilityFairValueGrossAsset" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeGainLossOnDerivativeNet" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeNotionalAmount" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DerivativeCollateralPost" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-13-stockholders-equity-details-textual" xlink:href="istr-20251231.xsd#statement-note-13-stockholders-equity-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-13-stockholders-equity-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_PreferredStockConvertibleForcedConversionTriggerPrice" xlink:label="istr_PreferredStockConvertibleForcedConversionTriggerPrice" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_PreferredStockConvertibleThresholdConsecutiveTradingDays" xlink:label="istr_PreferredStockConvertibleThresholdConsecutiveTradingDays" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_PreferredStockConvertibleThresholdTradingDays" xlink:label="istr_PreferredStockConvertibleThresholdTradingDays" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_ProceedsFromIssuanceOfPrivatePlacementNet" xlink:label="istr_ProceedsFromIssuanceOfPrivatePlacementNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember" xlink:label="us-gaap_CommonStockMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="us-gaap_CommonStockSharesAuthorized" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="us-gaap_CommonStockSharesOutstanding" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion" xlink:label="us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockConvertibleConversionRatio" xlink:label="us-gaap_PreferredStockConvertibleConversionRatio" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockDividendRatePercentage" xlink:label="us-gaap_PreferredStockDividendRatePercentage" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockLiquidationPreference" xlink:label="us-gaap_PreferredStockLiquidationPreference" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockRedemptionPricePerShare" xlink:label="us-gaap_PreferredStockRedemptionPricePerShare" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="us-gaap_PreferredStockSharesAuthorized" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:label="us-gaap_PreferredStockSharesOutstanding" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrivatePlacementMember" xlink:label="us-gaap_PrivatePlacementMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfPrivatePlacement" xlink:label="us-gaap_ProceedsFromIssuanceOfPrivatePlacement" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfCapitalizationEquityTable" xlink:label="us-gaap_ScheduleOfCapitalizationEquityTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesAPreferredStockMember" xlink:label="us-gaap_SeriesAPreferredStockMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharesIssuedPricePerShare" xlink:label="us-gaap_SharesIssuedPricePerShare" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="us-gaap_StatementEquityComponentsAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedDuringPeriodShares" xlink:label="us-gaap_StockRepurchasedDuringPeriodShares" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="us-gaap_SubsidiarySaleOfStockAxis" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfCapitalizationEquityTable" xlink:to="us-gaap_StatementClassOfStockAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_SeriesAPreferredStockMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfCapitalizationEquityTable" xlink:to="us-gaap_SubsidiarySaleOfStockAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="us-gaap_PrivatePlacementMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfCapitalizationEquityTable" xlink:to="us-gaap_StatementEquityComponentsAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_CommonStockMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfCapitalizationEquityTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfCapitalizationEquityTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockSharesAuthorized" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharesIssuedPricePerShare" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ProceedsFromIssuanceOfPrivatePlacement" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_ProceedsFromIssuanceOfPrivatePlacementNet" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockSharesOutstanding" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockDividendRatePercentage" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockLiquidationPreference" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockConvertibleConversionRatio" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_PreferredStockConvertibleThresholdTradingDays" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_PreferredStockConvertibleThresholdConsecutiveTradingDays" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_PreferredStockConvertibleForcedConversionTriggerPrice" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockRedemptionPricePerShare" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesAuthorized" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesOutstanding" xlink:type="arc"/>
    <link:definitionArc order="17" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockRepurchasedDuringPeriodShares" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-details-textual" xlink:href="istr-20251231.xsd#statement-note-14-stockbased-compensation-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_A2017LongTermIncentiveCompensationPlanMember" xlink:label="istr_A2017LongTermIncentiveCompensationPlanMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_EmployeesAndDirectorsMember" xlink:label="istr_EmployeesAndDirectorsMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_EmployeesMember" xlink:label="istr_EmployeesMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_KeyPersonnelMember" xlink:label="istr_KeyPersonnelMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_NonemployeeDirectorsAndCertainOfficersMember" xlink:label="istr_NonemployeeDirectorsAndCertainOfficersMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_RestrictedStockAndRestrictedStockUnitsMember" xlink:label="istr_RestrictedStockAndRestrictedStockUnitsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_DirectorMember" xlink:label="srt_DirectorMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualAxis" xlink:label="srt_TitleOfIndividualAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="srt_TitleOfIndividualWithRelationshipToEntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="us-gaap_EmployeeStockOptionMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameAxis" xlink:label="us-gaap_PlanNameAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain" xlink:label="us-gaap_PlanNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain" xlink:label="us-gaap_PlanNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="us-gaap_RestrictedStockUnitsRSUMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember" xlink:label="us-gaap_ShareBasedCompensationAwardTrancheOneMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheTwoMember" xlink:label="us-gaap_ShareBasedCompensationAwardTrancheTwoMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockOptionPlanExpense" xlink:label="us-gaap_StockOptionPlanExpense" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingAxis" xlink:label="us-gaap_VestingAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain" xlink:label="us-gaap_VestingDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain" xlink:label="us-gaap_VestingDomain-default" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_VestingAxis" xlink:to="us-gaap_VestingDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_TitleOfIndividualAxis" xlink:to="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_PlanNameAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PlanNameDomain" xlink:to="istr_A2017LongTermIncentiveCompensationPlanMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="srt_TitleOfIndividualAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="istr_KeyPersonnelMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_TitleOfIndividualAxis" xlink:to="srt_TitleOfIndividualWithRelationshipToEntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_AwardTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_EmployeeStockOptionMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_VestingAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VestingDomain" xlink:to="us-gaap_ShareBasedCompensationAwardTrancheOneMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_VestingAxis" xlink:to="us-gaap_VestingDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_RestrictedStockUnitsRSUMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="istr_NonemployeeDirectorsAndCertainOfficersMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="srt_RangeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="istr_EmployeesMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="srt_DirectorMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="istr_EmployeesAndDirectorsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VestingDomain" xlink:to="us-gaap_ShareBasedCompensationAwardTrancheTwoMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="istr_RestrictedStockAndRestrictedStockUnitsMember" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockOptionPlanExpense" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AllocatedShareBasedCompensationExpense" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-15-employee-benefits-details-textual" xlink:href="istr-20251231.xsd#statement-note-15-employee-benefits-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-15-employee-benefits-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_AccruedTaxesAndOtherLiabilitiesMember" xlink:label="istr_AccruedTaxesAndOtherLiabilitiesMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_CheahaFinancialGroupMember" xlink:label="istr_CheahaFinancialGroupMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DeferredCompensationArrangementWithIndividualMonthlyDistribution" xlink:label="istr_DeferredCompensationArrangementWithIndividualMonthlyDistribution" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DeferredCompensationArrangementWithIndividualReducedPaymentsMaximumAgeUponTermination" xlink:label="istr_DeferredCompensationArrangementWithIndividualReducedPaymentsMaximumAgeUponTermination" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DeferredCompensationArrangementWithIndividualRequisiteAgeOfEmployees" xlink:label="istr_DeferredCompensationArrangementWithIndividualRequisiteAgeOfEmployees" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DefinedBenefitPlanEligibilityForParticipationRequisiteAgeOfEmployees" xlink:label="istr_DefinedBenefitPlanEligibilityForParticipationRequisiteAgeOfEmployees" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DefinedContributionPlanAwardRequisiteServicePeriod" xlink:label="istr_DefinedContributionPlanAwardRequisiteServicePeriod" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DefinedContributionPlanEmployerDiscretionaryContributionAmountAwardVestingPeriod" xlink:label="istr_DefinedContributionPlanEmployerDiscretionaryContributionAmountAwardVestingPeriod" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DefinedContributionPlanEmployerDiscretionaryContributionAmountTotalAwardVestingPeriod" xlink:label="istr_DefinedContributionPlanEmployerDiscretionaryContributionAmountTotalAwardVestingPeriod" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DefinedContributionPlanEmployersDiscretionaryContributionAnnualVestingPercentage" xlink:label="istr_DefinedContributionPlanEmployersDiscretionaryContributionAnnualVestingPercentage" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_MinimumServicePeriodForParticipationInDefinedBenefitPlan" xlink:label="istr_MinimumServicePeriodForParticipationInDefinedBenefitPlan" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_SalariesAndEmployeeBenefitsMember" xlink:label="istr_SalariesAndEmployeeBenefitsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="us-gaap_BalanceSheetLocationAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="us-gaap_BalanceSheetLocationDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="us-gaap_BalanceSheetLocationDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="us-gaap_BusinessAcquisitionAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredCompensationArrangementWithIndividualCompensationExpense" xlink:label="us-gaap_DeferredCompensationArrangementWithIndividualCompensationExpense" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredCompensationArrangementWithIndividualMaximumContractualTerm1" xlink:label="us-gaap_DeferredCompensationArrangementWithIndividualMaximumContractualTerm1" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent" xlink:label="us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DefinedContributionPlanCostRecognized" xlink:label="us-gaap_DefinedContributionPlanCostRecognized" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount" xlink:label="us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent" xlink:label="us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralInsuranceExpense" xlink:label="us-gaap_GeneralInsuranceExpense" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="us-gaap_IncomeStatementLocationAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="us-gaap_IncomeStatementLocationDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="us-gaap_IncomeStatementLocationDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:label="us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SelfInsuranceReserve" xlink:label="us-gaap_SelfInsuranceReserve" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BalanceSheetLocationAxis" xlink:to="us-gaap_BalanceSheetLocationDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:to="us-gaap_BalanceSheetLocationAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BalanceSheetLocationDomain" xlink:to="istr_AccruedTaxesAndOtherLiabilitiesMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_BalanceSheetLocationAxis" xlink:to="us-gaap_BalanceSheetLocationDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:to="us-gaap_IncomeStatementLocationAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeStatementLocationDomain" xlink:to="istr_SalariesAndEmployeeBenefitsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:to="us-gaap_BusinessAcquisitionAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="istr_CheahaFinancialGroupMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SelfInsuranceReserve" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_GeneralInsuranceExpense" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DefinedBenefitPlanEligibilityForParticipationRequisiteAgeOfEmployees" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_MinimumServicePeriodForParticipationInDefinedBenefitPlan" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DefinedContributionPlanCostRecognized" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DefinedContributionPlanEmployersDiscretionaryContributionAnnualVestingPercentage" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DefinedContributionPlanEmployerDiscretionaryContributionAmountAwardVestingPeriod" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DefinedContributionPlanAwardRequisiteServicePeriod" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DefinedContributionPlanEmployerDiscretionaryContributionAmountTotalAwardVestingPeriod" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DeferredCompensationArrangementWithIndividualRequisiteAgeOfEmployees" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredCompensationArrangementWithIndividualMaximumContractualTerm1" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DeferredCompensationArrangementWithIndividualReducedPaymentsMaximumAgeUponTermination" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DeferredCompensationArrangementWithIndividualMonthlyDistribution" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent" xlink:type="arc"/>
    <link:definitionArc order="17" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredCompensationArrangementWithIndividualCompensationExpense" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes-details-textual" xlink:href="istr-20251231.xsd#statement-note-16-income-taxes-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DomesticCountryMember" xlink:label="us-gaap_DomesticCountryMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:label="us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:label="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:label="us-gaap_IncomeTaxAuthorityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="us-gaap_IncomeTaxAuthorityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="us-gaap_IncomeTaxAuthorityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_IncomeTaxAuthorityAxis" xlink:to="us-gaap_IncomeTaxAuthorityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:to="us-gaap_IncomeTaxAuthorityAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeTaxAuthorityDomain" xlink:to="us-gaap_DomesticCountryMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_IncomeTaxAuthorityAxis" xlink:to="us-gaap_IncomeTaxAuthorityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments-details-textual" xlink:href="istr-20251231.xsd#statement-note-17-fair-values-of-financial-instruments-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_FinancingReceivableImpairedDuringPeriodAllowanceForCreditLossesIndividuallyEvaluatedForImpairment" xlink:label="istr_FinancingReceivableImpairedDuringPeriodAllowanceForCreditLossesIndividuallyEvaluatedForImpairment" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_FinancingReceivableImpairedDuringPeriodGross" xlink:label="istr_FinancingReceivableImpairedDuringPeriodGross" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OtherRealEstateOwnedFairValueDisclosure" xlink:label="istr_OtherRealEstateOwnedFairValueDisclosure" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OtherRealEstateOwnedWriteDowns" xlink:label="istr_OtherRealEstateOwnedWriteDowns" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="us-gaap_BalanceSheetLocationAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="us-gaap_BalanceSheetLocationDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="us-gaap_BalanceSheetLocationDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:label="us-gaap_FairValueByMeasurementFrequencyAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="us-gaap_FairValueInputsLevel3Member" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasuredAtNetAssetValuePerShareMember" xlink:label="us-gaap_FairValueMeasuredAtNetAssetValuePerShareMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="us-gaap_FairValueMeasurementFrequencyDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="us-gaap_FairValueMeasurementFrequencyDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1" xlink:label="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue" xlink:label="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsNonrecurringMember" xlink:label="us-gaap_FairValueMeasurementsNonrecurringMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialLiabilitiesFairValueDisclosure" xlink:label="us-gaap_FinancialLiabilitiesFairValueDisclosure" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentsFairValueDisclosure" xlink:label="us-gaap_InvestmentsFairValueDisclosure" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssetsMember" xlink:label="us-gaap_OtherAssetsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BalanceSheetLocationAxis" xlink:to="us-gaap_BalanceSheetLocationDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByMeasurementFrequencyAxis" xlink:to="us-gaap_FairValueMeasurementFrequencyDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:to="us-gaap_BalanceSheetLocationAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BalanceSheetLocationDomain" xlink:to="us-gaap_OtherAssetsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_BalanceSheetLocationAxis" xlink:to="us-gaap_BalanceSheetLocationDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:to="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueMeasuredAtNetAssetValuePerShareMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:to="us-gaap_FairValueByMeasurementFrequencyAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementFrequencyDomain" xlink:to="us-gaap_FairValueMeasurementsNonrecurringMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByMeasurementFrequencyAxis" xlink:to="us-gaap_FairValueMeasurementFrequencyDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel3Member" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_InvestmentsFairValueDisclosure" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancialLiabilitiesFairValueDisclosure" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_FinancingReceivableImpairedDuringPeriodGross" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_FinancingReceivableImpairedDuringPeriodAllowanceForCreditLossesIndividuallyEvaluatedForImpairment" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_OtherRealEstateOwnedFairValueDisclosure" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_OtherRealEstateOwnedWriteDowns" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments-reconciliation-for-assets-measured-at-fair-value-on-recurring-basis-using-significant-unobservable-inputs-details-parentheticals" xlink:href="istr-20251231.xsd#statement-note-17-fair-values-of-financial-instruments-reconciliation-for-assets-measured-at-fair-value-on-recurring-basis-using-significant-unobservable-inputs-details-parentheticals" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments-reconciliation-for-assets-measured-at-fair-value-on-recurring-basis-using-significant-unobservable-inputs-details-parentheticals" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:label="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueRecurringBasisUnobservableInputReconciliationAssetGainLossStatementOfOtherComprehensiveIncomeExtensibleList" xlink:label="us-gaap_FairValueRecurringBasisUnobservableInputReconciliationAssetGainLossStatementOfOtherComprehensiveIncomeExtensibleList" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueRecurringBasisUnobservableInputReconciliationAssetGainLossStatementOfOtherComprehensiveIncomeExtensibleList" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-18-regulatory-matters-details-textual" xlink:href="istr-20251231.xsd#statement-note-18-regulatory-matters-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-18-regulatory-matters-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramTable" xlink:label="srt_ShareRepurchaseProgramTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BankingRegulationCapitalConservationBufferCapitalConservedMinimum" xlink:label="us-gaap_BankingRegulationCapitalConservationBufferCapitalConservedMinimum" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="srt_ShareRepurchaseProgramTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="srt_ShareRepurchaseProgramTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_BankingRegulationCapitalConservationBufferCapitalConservedMinimum" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-19-commitments-and-contingencies-details-textual" xlink:href="istr-20251231.xsd#statement-note-19-commitments-and-contingencies-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-19-commitments-and-contingencies-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_CommitmentToFundInvestmentInSBICQualifiedFunds" xlink:label="istr_CommitmentToFundInvestmentInSBICQualifiedFunds" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitmentsTable" xlink:label="us-gaap_OtherCommitmentsTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_OtherCommitmentsTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherCommitmentsTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_CommitmentToFundInvestmentInSBICQualifiedFunds" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-23-subsequent-events-details-textual" xlink:href="istr-20251231.xsd#statement-note-23-subsequent-events-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-23-subsequent-events-details-textual" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_WFBMember" xlink:label="istr_WFBMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Assets" xlink:label="us-gaap_Assets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="us-gaap_BusinessAcquisitionAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued" xlink:label="us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Deposits" xlink:label="us-gaap_Deposits" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotesReceivableNet" xlink:label="us-gaap_NotesReceivableNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireBusinessesGross" xlink:label="us-gaap_PaymentsToAcquireBusinessesGross" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventMember" xlink:label="us-gaap_SubsequentEventMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTable" xlink:label="us-gaap_SubsequentEventTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="us-gaap_SubsequentEventTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="us-gaap_SubsequentEventTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="us-gaap_SubsequentEventTypeDomain-default" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_SubsequentEventTypeAxis" xlink:to="us-gaap_SubsequentEventTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_SubsequentEventTable" xlink:to="us-gaap_BusinessAcquisitionAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="istr_WFBMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_SubsequentEventTable" xlink:to="us-gaap_SubsequentEventTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SubsequentEventTypeDomain" xlink:to="us-gaap_SubsequentEventMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_SubsequentEventTypeAxis" xlink:to="us-gaap_SubsequentEventTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_SubsequentEventTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_EntityDomain" xlink:to="istr_WFBMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SubsequentEventTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PaymentsToAcquireBusinessesGross" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_Assets" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NotesReceivableNet" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_Deposits" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-6-leases-future-minimum-lease-payment-details" xlink:href="istr-20251231.xsd#statement-note-6-leases-future-minimum-lease-payment-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-6-leases-future-minimum-lease-payment-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeLeaseDescriptionTable" xlink:label="us-gaap_LesseeLeaseDescriptionTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFive" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFive" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFour" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFour" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearThree" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearThree" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearTwo" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearTwo" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseLiabilityUndiscountedExcessAmount" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityUndiscountedExcessAmount" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiability" xlink:label="us-gaap_OperatingLeaseLiability" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_LesseeLeaseDescriptionTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LesseeLeaseDescriptionTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearTwo" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearThree" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFour" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFive" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LesseeOperatingLeaseLiabilityUndiscountedExcessAmount" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeaseLiability" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-7-goodwill-and-other-intangible-assets-amortization-schedule-for-core-deposit-intangible-assets-details" xlink:href="istr-20251231.xsd#statement-note-7-goodwill-and-other-intangible-assets-amortization-schedule-for-core-deposit-intangible-assets-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-7-goodwill-and-other-intangible-assets-amortization-schedule-for-core-deposit-intangible-assets-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CoreDepositsMember" xlink:label="us-gaap_CoreDepositsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsNet" xlink:label="us-gaap_FiniteLivedIntangibleAssetsNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:label="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_CoreDepositsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsNet" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-8-deposits-scheduled-maturates-of-time-deposits-details" xlink:href="istr-20251231.xsd#statement-note-8-deposits-scheduled-maturates-of-time-deposits-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-8-deposits-scheduled-maturates-of-time-deposits-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShortTermDebtTable" xlink:label="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimeDepositMaturitiesYearFive" xlink:label="us-gaap_TimeDepositMaturitiesYearFive" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimeDepositMaturitiesYearFour" xlink:label="us-gaap_TimeDepositMaturitiesYearFour" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimeDepositMaturitiesYearOne" xlink:label="us-gaap_TimeDepositMaturitiesYearOne" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimeDepositMaturitiesYearThree" xlink:label="us-gaap_TimeDepositMaturitiesYearThree" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimeDepositMaturitiesYearTwo" xlink:label="us-gaap_TimeDepositMaturitiesYearTwo" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimeDeposits" xlink:label="us-gaap_TimeDeposits" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TimeDepositMaturitiesYearOne" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TimeDepositMaturitiesYearTwo" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TimeDepositMaturitiesYearThree" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TimeDepositMaturitiesYearFour" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TimeDepositMaturitiesYearFive" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TimeDeposits" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-consolidated-statements-of-changes-in-stockholders-equity" xlink:href="istr-20251231.xsd#statement-consolidated-statements-of-changes-in-stockholders-equity" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-consolidated-statements-of-changes-in-stockholders-equity" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountingStandardsUpdate201613Member" xlink:label="us-gaap_AccountingStandardsUpdate201613Member" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="us-gaap_AdditionalPaidInCapitalMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:label="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation" xlink:label="us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember" xlink:label="us-gaap_CommonStockMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsCommonStockCash" xlink:label="us-gaap_DividendsCommonStockCash" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsPreferredStock" xlink:label="us-gaap_DividendsPreferredStock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" xlink:label="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockMember" xlink:label="us-gaap_PreferredStockMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember" xlink:label="us-gaap_RetainedEarningsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="us-gaap_StatementEquityComponentsAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation" xlink:label="us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:label="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue" xlink:label="us-gaap_StockRepurchasedAndRetiredDuringPeriodValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity" xlink:label="us-gaap_StockholdersEquity" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="us-gaap_TypeOfAdoptionMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="us-gaap_TypeOfAdoptionMember-default" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TypeOfAdoptionMember" xlink:to="us-gaap_AccountingStandardsUpdate201613Member" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:to="us-gaap_TypeOfAdoptionMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementEquityComponentsAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_PreferredStockMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_CommonStockMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_AdditionalPaidInCapitalMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_RetainedEarningsMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:to="us-gaap_TypeOfAdoptionMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockholdersEquity" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockRepurchasedAndRetiredDuringPeriodValue" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DividendsCommonStockCash" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetIncomeLoss" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DividendsPreferredStock" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-amortized-cost-and-fair-value-of-investment-securities-classified-as-afs-details" xlink:href="istr-20251231.xsd#statement-note-2-investment-securities-amortized-cost-and-fair-value-of-investment-securities-classified-as-afs-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-amortized-cost-and-fair-value-of-investment-securities-classified-as-afs-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" xlink:label="us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" xlink:label="us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialMortgageBackedSecuritiesMember" xlink:label="us-gaap_CommercialMortgageBackedSecuritiesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateDebtSecuritiesMember" xlink:label="us-gaap_CorporateDebtSecuritiesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResidentialMortgageBackedSecuritiesMember" xlink:label="us-gaap_ResidentialMortgageBackedSecuritiesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember" xlink:label="us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:label="us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtSecuritiesAvailableForSaleTable" xlink:to="us-gaap_FinancialInstrumentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_CorporateDebtSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_ResidentialMortgageBackedSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_CommercialMortgageBackedSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtSecuritiesAvailableForSaleTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-amortized-cost-and-fair-value-of-investment-securities-classified-as-htm-details" xlink:href="istr-20251231.xsd#statement-note-2-investment-securities-amortized-cost-and-fair-value-of-investment-securities-classified-as-htm-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-amortized-cost-and-fair-value-of-investment-securities-classified-as-htm-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:label="us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain" xlink:label="us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss" xlink:label="us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesFairValue" xlink:label="us-gaap_HeldToMaturitySecuritiesFairValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResidentialMortgageBackedSecuritiesMember" xlink:label="us-gaap_ResidentialMortgageBackedSecuritiesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfHeldToMaturitySecuritiesTable" xlink:label="us-gaap_ScheduleOfHeldToMaturitySecuritiesTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:label="us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfHeldToMaturitySecuritiesTable" xlink:to="us-gaap_FinancialInstrumentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_ResidentialMortgageBackedSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfHeldToMaturitySecuritiesTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfHeldToMaturitySecuritiesTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_HeldToMaturitySecuritiesFairValue" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-11-other-borrowing-funds-junior-subordinated-debt-details" xlink:href="istr-20251231.xsd#statement-note-11-other-borrowing-funds-junior-subordinated-debt-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-11-other-borrowing-funds-junior-subordinated-debt-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_BOJBancsharesStatutoryTrustIMember" xlink:label="istr_BOJBancsharesStatutoryTrustIMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_CheahaStatutoryTrustIMember" xlink:label="istr_CheahaStatutoryTrustIMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_FirstCommunityLouisianaStatutoryTrustIMember" xlink:label="istr_FirstCommunityLouisianaStatutoryTrustIMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis" xlink:label="us-gaap_DebtInstrumentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" xlink:label="us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable" xlink:label="us-gaap_DebtInstrumentTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust" xlink:label="us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebt" xlink:label="us-gaap_LongTermDebt" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_DebtInstrumentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="istr_BOJBancsharesStatutoryTrustIMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="istr_CheahaStatutoryTrustIMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="istr_FirstCommunityLouisianaStatutoryTrustIMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtInstrumentTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LongTermDebt" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-13-stockholders-equity-accumulated-other-comprehensive-income-loss-details" xlink:href="istr-20251231.xsd#statement-note-13-stockholders-equity-accumulated-other-comprehensive-income-loss-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-13-stockholders-equity-accumulated-other-comprehensive-income-loss-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember" xlink:label="us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OciBeforeReclassificationsNetOfTaxAttributableToParent" xlink:label="us-gaap_OciBeforeReclassificationsNetOfTaxAttributableToParent" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReclassificationFromAociCurrentPeriodNetOfTaxAttributableToParent" xlink:label="us-gaap_ReclassificationFromAociCurrentPeriodNetOfTaxAttributableToParent" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable" xlink:label="us-gaap_ScheduleOfStockByClassTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="us-gaap_StatementEquityComponentsAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity" xlink:label="us-gaap_StockholdersEquity" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfStockByClassTable" xlink:to="us-gaap_StatementEquityComponentsAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfStockByClassTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfStockByClassTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockholdersEquity" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OciBeforeReclassificationsNetOfTaxAttributableToParent" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ReclassificationFromAociCurrentPeriodNetOfTaxAttributableToParent" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-schedule-of-stock-option-activity-details" xlink:href="istr-20251231.xsd#statement-note-14-stockbased-compensation-schedule-of-stock-option-activity-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-schedule-of-stock-option-activity-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-schedule-of-stock-option-valuation-assumptions-details" xlink:href="istr-20251231.xsd#statement-note-14-stockbased-compensation-schedule-of-stock-option-valuation-assumptions-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-schedule-of-stock-option-valuation-assumptions-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments-reconciliation-for-assets-measured-at-fair-value-on-recurring-basis-using-significant-unobservable-inputs-details" xlink:href="istr-20251231.xsd#statement-note-17-fair-values-of-financial-instruments-reconciliation-for-assets-measured-at-fair-value-on-recurring-basis-using-significant-unobservable-inputs-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments-reconciliation-for-assets-measured-at-fair-value-on-recurring-basis-using-significant-unobservable-inputs-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateDebtSecuritiesMember" xlink:label="us-gaap_CorporateDebtSecuritiesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:label="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1" xlink:label="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss" xlink:label="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases" xlink:label="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales" xlink:label="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements" xlink:label="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3" xlink:label="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3" xlink:label="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue" xlink:label="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:label="us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:to="us-gaap_FinancialInstrumentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_CorporateDebtSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-19-commitments-and-contingencies-commitments-to-extend-credit-details" xlink:href="istr-20251231.xsd#statement-note-19-commitments-and-contingencies-commitments-to-extend-credit-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-19-commitments-and-contingencies-commitments-to-extend-credit-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommitmentsToExtendCreditMember" xlink:label="us-gaap_CommitmentsToExtendCreditMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesByNatureOfContingencyAxis" xlink:label="us-gaap_LossContingenciesByNatureOfContingencyAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingencyNatureDomain" xlink:label="us-gaap_LossContingencyNatureDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingencyNatureDomain" xlink:label="us-gaap_LossContingencyNatureDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitmentsTable" xlink:label="us-gaap_OtherCommitmentsTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StandbyLettersOfCreditMember" xlink:label="us-gaap_StandbyLettersOfCreditMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnusedCommitmentsToExtendCredit" xlink:label="us-gaap_UnusedCommitmentsToExtendCredit" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_LossContingenciesByNatureOfContingencyAxis" xlink:to="us-gaap_LossContingencyNatureDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_OtherCommitmentsTable" xlink:to="us-gaap_LossContingenciesByNatureOfContingencyAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LossContingencyNatureDomain" xlink:to="us-gaap_CommitmentsToExtendCreditMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_LossContingenciesByNatureOfContingencyAxis" xlink:to="us-gaap_LossContingencyNatureDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LossContingencyNatureDomain" xlink:to="us-gaap_StandbyLettersOfCreditMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_OtherCommitmentsTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherCommitmentsTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_UnusedCommitmentsToExtendCredit" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-19-commitments-and-contingencies-summary-of-acl-on-unfunded-loan-commitments-details" xlink:href="istr-20251231.xsd#statement-note-19-commitments-and-contingencies-summary-of-acl-on-unfunded-loan-commitments-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-19-commitments-and-contingencies-summary-of-acl-on-unfunded-loan-commitments-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember" xlink:label="srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionAxis" xlink:label="srt_CumulativeEffectPeriodOfAdoptionAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain" xlink:label="srt_CumulativeEffectPeriodOfAdoptionDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain" xlink:label="srt_CumulativeEffectPeriodOfAdoptionDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountingStandardsUpdate201613Member" xlink:label="us-gaap_AccountingStandardsUpdate201613Member" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:label="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OffBalanceSheetCreditLossLiability" xlink:label="us-gaap_OffBalanceSheetCreditLossLiability" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal" xlink:label="us-gaap_OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFairValueOffBalanceSheetRisksTable" xlink:label="us-gaap_ScheduleOfFairValueOffBalanceSheetRisksTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="us-gaap_TypeOfAdoptionMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="us-gaap_TypeOfAdoptionMember-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnfundedLoanCommitmentMember" xlink:label="us-gaap_UnfundedLoanCommitmentMember" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:to="us-gaap_TypeOfAdoptionMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_CumulativeEffectPeriodOfAdoptionAxis" xlink:to="srt_CumulativeEffectPeriodOfAdoptionDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFairValueOffBalanceSheetRisksTable" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="us-gaap_UnfundedLoanCommitmentMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFairValueOffBalanceSheetRisksTable" xlink:to="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TypeOfAdoptionMember" xlink:to="us-gaap_AccountingStandardsUpdate201613Member" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:to="us-gaap_TypeOfAdoptionMember-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFairValueOffBalanceSheetRisksTable" xlink:to="srt_CumulativeEffectPeriodOfAdoptionAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_CumulativeEffectPeriodOfAdoptionDomain" xlink:to="srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_CumulativeEffectPeriodOfAdoptionAxis" xlink:to="srt_CumulativeEffectPeriodOfAdoptionDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFairValueOffBalanceSheetRisksTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfFairValueOffBalanceSheetRisksTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OffBalanceSheetCreditLossLiability" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-consolidated-balance-sheets" xlink:href="istr-20251231.xsd#statement-consolidated-balance-sheets" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-consolidated-balance-sheets" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_EquitySecuritiesHeldInCorrespondentBanksMember" xlink:label="istr_EquitySecuritiesHeldInCorrespondentBanksMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_MarketableSecuritiesAndMutualFundsMember" xlink:label="istr_MarketableSecuritiesAndMutualFundsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent" xlink:label="us-gaap_AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapital" xlink:label="us-gaap_AdditionalPaidInCapital" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdvancesFromFederalHomeLoanBanks" xlink:label="us-gaap_AdvancesFromFederalHomeLoanBanks" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Assets" xlink:label="us-gaap_Assets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract" xlink:label="us-gaap_AssetsAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BankOwnedLifeInsurance" xlink:label="us-gaap_BankOwnedLifeInsurance" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndDueFromBanks" xlink:label="us-gaap_CashAndDueFromBanks" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsAndFederalFundsSold" xlink:label="us-gaap_CashCashEquivalentsAndFederalFundsSold" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommitmentsAndContingencies" xlink:label="us-gaap_CommitmentsAndContingencies" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockValue" xlink:label="us-gaap_CommonStockValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:label="us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxAssetsNet" xlink:label="us-gaap_DeferredIncomeTaxAssetsNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Deposits" xlink:label="us-gaap_Deposits" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepositsAbstract" xlink:label="us-gaap_DepositsAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent" xlink:label="us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalFundsPurchasedAndSecuritiesSoldUnderAgreementsToRepurchase" xlink:label="us-gaap_FederalFundsPurchasedAndSecuritiesSoldUnderAgreementsToRepurchase" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest" xlink:label="us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestAfterAllowanceForCreditLoss" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestAfterAllowanceForCreditLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeclosedAssets" xlink:label="us-gaap_ForeclosedAssets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IntangibleAssetsNetIncludingGoodwill" xlink:label="us-gaap_IntangibleAssetsNetIncludingGoodwill" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestBearingDepositLiabilities" xlink:label="us-gaap_InterestBearingDepositLiabilities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestBearingDepositsInBanks" xlink:label="us-gaap_InterestBearingDepositsInBanks" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestReceivable" xlink:label="us-gaap_InterestReceivable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust" xlink:label="us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Liabilities" xlink:label="us-gaap_Liabilities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquity" xlink:label="us-gaap_LiabilitiesAndStockholdersEquity" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoninterestBearingDepositLiabilities" xlink:label="us-gaap_NoninterestBearingDepositLiabilities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssets" xlink:label="us-gaap_OtherAssets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockValue" xlink:label="us-gaap_PreferredStockValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="us-gaap_PropertyPlantAndEquipmentNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsAccumulatedDeficit" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity" xlink:label="us-gaap_StockholdersEquity" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityAbstract" xlink:label="us-gaap_StockholdersEquityAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedDebt" xlink:label="us-gaap_SubordinatedDebt" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_FinancialInstrumentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="istr_MarketableSecuritiesAndMutualFundsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="istr_EquitySecuritiesHeldInCorrespondentBanksMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AssetsAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_CashAndDueFromBanks" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_InterestBearingDepositsInBanks" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_CashCashEquivalentsAndFederalFundsSold" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestAfterAllowanceForCreditLoss" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_PropertyPlantAndEquipmentNet" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_ForeclosedAssets" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_InterestReceivable" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_DeferredIncomeTaxAssetsNet" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_IntangibleAssetsNetIncludingGoodwill" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_BankOwnedLifeInsurance" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_OtherAssets" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_Assets" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DepositsAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DepositsAbstract" xlink:to="us-gaap_NoninterestBearingDepositLiabilities" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DepositsAbstract" xlink:to="us-gaap_InterestBearingDepositLiabilities" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DepositsAbstract" xlink:to="us-gaap_Deposits" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DepositsAbstract" xlink:to="us-gaap_AdvancesFromFederalHomeLoanBanks" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DepositsAbstract" xlink:to="us-gaap_FederalFundsPurchasedAndSecuritiesSoldUnderAgreementsToRepurchase" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DepositsAbstract" xlink:to="us-gaap_SubordinatedDebt" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DepositsAbstract" xlink:to="us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DepositsAbstract" xlink:to="us-gaap_AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DepositsAbstract" xlink:to="us-gaap_Liabilities" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DepositsAbstract" xlink:to="us-gaap_CommitmentsAndContingencies" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockholdersEquityAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_PreferredStockValue" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_CommonStockValue" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_AdditionalPaidInCapital" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_RetainedEarningsAccumulatedDeficit" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_StockholdersEquity" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_LiabilitiesAndStockholdersEquity" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-consolidated-statements-of-income" xlink:href="istr-20251231.xsd#statement-consolidated-statements-of-income" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-consolidated-statements-of-income" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis" xlink:label="srt_ProductOrServiceAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain" xlink:label="srt_ProductsAndServicesDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain" xlink:label="srt_ProductsAndServicesDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BankOwnedLifeInsuranceIncome" xlink:label="us-gaap_BankOwnedLifeInsuranceIncome" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BankServicingMember" xlink:label="us-gaap_BankServicingMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts" xlink:label="us-gaap_BusinessCombinationAcquisitionRelatedCosts" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditAndDebitCardMember" xlink:label="us-gaap_CreditAndDebitCardMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesRealizedGainLoss" xlink:label="us-gaap_DebtSecuritiesRealizedGainLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepositAccountMember" xlink:label="us-gaap_DepositAccountMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationAndAmortization" xlink:label="us-gaap_DepreciationAndAmortization" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareAbstract" xlink:label="us-gaap_EarningsPerShareAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasic" xlink:label="us-gaap_EarningsPerShareBasic" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDiluted" xlink:label="us-gaap_EarningsPerShareDiluted" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss" xlink:label="us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnSaleOfPropertyPlantEquipment" xlink:label="us-gaap_GainLossOnSaleOfPropertyPlantEquipment" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnSalesOfLoansNet" xlink:label="us-gaap_GainLossOnSalesOfLoansNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossRelatedToLitigationSettlement" xlink:label="us-gaap_GainLossRelatedToLitigationSettlement" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt" xlink:label="us-gaap_GainsLossesOnExtinguishmentOfDebt" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainsLossesOnSalesOfOtherRealEstate" xlink:label="us-gaap_GainsLossesOnSalesOfOtherRealEstate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" xlink:label="us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="us-gaap_IncomeTaxExpenseBenefit" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InformationTechnologyAndDataProcessing" xlink:label="us-gaap_InformationTechnologyAndDataProcessing" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestAndDividendIncomeOperating" xlink:label="us-gaap_InterestAndDividendIncomeOperating" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestAndDividendIncomeOperatingAbstract" xlink:label="us-gaap_InterestAndDividendIncomeOperatingAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestAndDividendIncomeSecuritiesByTaxableStatusAbstract" xlink:label="us-gaap_InterestAndDividendIncomeSecuritiesByTaxableStatusAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestAndFeeIncomeLoansAndLeases" xlink:label="us-gaap_InterestAndFeeIncomeLoansAndLeases" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseAbstract" xlink:label="us-gaap_InterestExpenseAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseBorrowings" xlink:label="us-gaap_InterestExpenseBorrowings" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseDeposits" xlink:label="us-gaap_InterestExpenseDeposits" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseOperating" xlink:label="us-gaap_InterestExpenseOperating" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestIncomeExpenseAfterProvisionForLoanLoss" xlink:label="us-gaap_InterestIncomeExpenseAfterProvisionForLoanLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestIncomeExpenseNet" xlink:label="us-gaap_InterestIncomeExpenseNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestIncomeOther" xlink:label="us-gaap_InterestIncomeOther" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestIncomeSecuritiesTaxExempt" xlink:label="us-gaap_InterestIncomeSecuritiesTaxExempt" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestIncomeSecuritiesTaxable" xlink:label="us-gaap_InterestIncomeSecuritiesTaxable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LaborAndRelatedExpense" xlink:label="us-gaap_LaborAndRelatedExpense" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MarketingExpense" xlink:label="us-gaap_MarketingExpense" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic" xlink:label="us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoninterestExpense" xlink:label="us-gaap_NoninterestExpense" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoninterestExpenseAbstract" xlink:label="us-gaap_NoninterestExpenseAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoninterestIncome" xlink:label="us-gaap_NoninterestIncome" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoninterestIncomeAbstract" xlink:label="us-gaap_NoninterestIncomeAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoninterestIncomeOtherOperatingIncome" xlink:label="us-gaap_NoninterestIncomeOtherOperatingIncome" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OccupancyNet" xlink:label="us-gaap_OccupancyNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNoninterestExpense" xlink:label="us-gaap_OtherNoninterestExpense" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockDividendsIncomeStatementImpact" xlink:label="us-gaap_PreferredStockDividendsIncomeStatementImpact" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfessionalFees" xlink:label="us-gaap_ProfessionalFees" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProvisionForLoanLeaseAndOtherLosses" xlink:label="us-gaap_ProvisionForLoanLeaseAndOtherLosses" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenuesNetOfInterestExpense" xlink:label="us-gaap_RevenuesNetOfInterestExpense" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_ProductOrServiceAxis" xlink:to="srt_ProductsAndServicesDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="srt_ProductOrServiceAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_ProductsAndServicesDomain" xlink:to="us-gaap_DepositAccountMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_ProductOrServiceAxis" xlink:to="srt_ProductsAndServicesDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_ProductsAndServicesDomain" xlink:to="us-gaap_BankServicingMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_ProductsAndServicesDomain" xlink:to="us-gaap_CreditAndDebitCardMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_InterestAndDividendIncomeOperatingAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InterestAndDividendIncomeOperatingAbstract" xlink:to="us-gaap_InterestAndFeeIncomeLoansAndLeases" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_InterestAndDividendIncomeSecuritiesByTaxableStatusAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InterestAndDividendIncomeSecuritiesByTaxableStatusAbstract" xlink:to="us-gaap_InterestIncomeSecuritiesTaxable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InterestAndDividendIncomeSecuritiesByTaxableStatusAbstract" xlink:to="us-gaap_InterestIncomeSecuritiesTaxExempt" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InterestAndDividendIncomeSecuritiesByTaxableStatusAbstract" xlink:to="us-gaap_InterestIncomeOther" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InterestAndDividendIncomeSecuritiesByTaxableStatusAbstract" xlink:to="us-gaap_InterestAndDividendIncomeOperating" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_InterestExpenseAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InterestExpenseAbstract" xlink:to="us-gaap_InterestExpenseDeposits" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InterestExpenseAbstract" xlink:to="us-gaap_InterestExpenseBorrowings" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InterestExpenseAbstract" xlink:to="us-gaap_InterestExpenseOperating" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InterestExpenseAbstract" xlink:to="us-gaap_InterestIncomeExpenseNet" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InterestExpenseAbstract" xlink:to="us-gaap_ProvisionForLoanLeaseAndOtherLosses" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InterestExpenseAbstract" xlink:to="us-gaap_InterestIncomeExpenseAfterProvisionForLoanLoss" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NoninterestIncomeAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestIncomeAbstract" xlink:to="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestIncomeAbstract" xlink:to="us-gaap_DebtSecuritiesRealizedGainLoss" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestIncomeAbstract" xlink:to="us-gaap_GainLossOnSaleOfPropertyPlantEquipment" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestIncomeAbstract" xlink:to="us-gaap_GainsLossesOnSalesOfOtherRealEstate" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestIncomeAbstract" xlink:to="us-gaap_GainLossOnSalesOfLoansNet" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestIncomeAbstract" xlink:to="us-gaap_BankOwnedLifeInsuranceIncome" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestIncomeAbstract" xlink:to="us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestIncomeAbstract" xlink:to="us-gaap_GainLossRelatedToLitigationSettlement" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestIncomeAbstract" xlink:to="us-gaap_NoninterestIncomeOtherOperatingIncome" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestIncomeAbstract" xlink:to="us-gaap_NoninterestIncome" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestIncomeAbstract" xlink:to="us-gaap_RevenuesNetOfInterestExpense" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NoninterestExpenseAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestExpenseAbstract" xlink:to="us-gaap_DepreciationAndAmortization" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestExpenseAbstract" xlink:to="us-gaap_LaborAndRelatedExpense" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestExpenseAbstract" xlink:to="us-gaap_OccupancyNet" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestExpenseAbstract" xlink:to="us-gaap_InformationTechnologyAndDataProcessing" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestExpenseAbstract" xlink:to="us-gaap_MarketingExpense" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestExpenseAbstract" xlink:to="us-gaap_ProfessionalFees" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestExpenseAbstract" xlink:to="us-gaap_GainsLossesOnExtinguishmentOfDebt" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestExpenseAbstract" xlink:to="us-gaap_BusinessCombinationAcquisitionRelatedCosts" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestExpenseAbstract" xlink:to="us-gaap_OtherNoninterestExpense" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestExpenseAbstract" xlink:to="us-gaap_NoninterestExpense" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestExpenseAbstract" xlink:to="us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestExpenseAbstract" xlink:to="us-gaap_IncomeTaxExpenseBenefit" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestExpenseAbstract" xlink:to="us-gaap_NetIncomeLoss" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestExpenseAbstract" xlink:to="us-gaap_PreferredStockDividendsIncomeStatementImpact" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoninterestExpenseAbstract" xlink:to="us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EarningsPerShareAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EarningsPerShareAbstract" xlink:to="us-gaap_EarningsPerShareBasic" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EarningsPerShareAbstract" xlink:to="us-gaap_EarningsPerShareDiluted" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-consolidated-statements-of-cash-flows" xlink:href="istr-20251231.xsd#statement-consolidated-statements-of-cash-flows" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-consolidated-statements-of-cash-flows" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_AmortizationOfPurchaseAccountingAdjustments" xlink:label="istr_AmortizationOfPurchaseAccountingAdjustments" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DistributionsFromInvestments" xlink:label="istr_DistributionsFromInvestments" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_EquitySecuritiesHeldInCorrespondentBanksMember" xlink:label="istr_EquitySecuritiesHeldInCorrespondentBanksMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_FederalHomeLoanBankStockDividends" xlink:label="istr_FederalHomeLoanBankStockDividends" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_GainLossOnSalesOfForeclosedAssets" xlink:label="istr_GainLossOnSalesOfForeclosedAssets" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_IncreaseDecreaseInShorttermFhlbAdvances" xlink:label="istr_IncreaseDecreaseInShorttermFhlbAdvances" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_MarketableSecuritiesAndMutualFundsMember" xlink:label="istr_MarketableSecuritiesAndMutualFundsMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_ProvisionForOtherRealEstateOwned" xlink:label="istr_ProvisionForOtherRealEstateOwned" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_TransferFromBankPremisesAndEquipmentToOtherAssets" xlink:label="istr_TransferFromBankPremisesAndEquipmentToOtherAssets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments" xlink:label="us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfFinancingCosts" xlink:label="us-gaap_AmortizationOfFinancingCosts" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesRealizedGainLoss" xlink:label="us-gaap_DebtSecuritiesRealizedGainLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxExpenseBenefit" xlink:label="us-gaap_DeferredIncomeTaxExpenseBenefit" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationAndAmortization" xlink:label="us-gaap_DepreciationAndAmortization" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss" xlink:label="us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnSaleOfPropertyPlantEquipment" xlink:label="us-gaap_GainLossOnSaleOfPropertyPlantEquipment" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnSalesOfLoansNet" xlink:label="us-gaap_GainLossOnSalesOfLoansNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt" xlink:label="us-gaap_GainsLossesOnExtinguishmentOfDebt" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidFederalBeforeRefundReceived" xlink:label="us-gaap_IncomeTaxPaidFederalBeforeRefundReceived" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidStateAndLocalBeforeRefundReceived" xlink:label="us-gaap_IncomeTaxPaidStateAndLocalBeforeRefundReceived" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxesPaid" xlink:label="us-gaap_IncomeTaxesPaid" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInAccruedInterestReceivableNet" xlink:label="us-gaap_IncreaseDecreaseInAccruedInterestReceivableNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInDeposits" xlink:label="us-gaap_IncreaseDecreaseInDeposits" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:label="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOtherAccruedLiabilities" xlink:label="us-gaap_IncreaseDecreaseInOtherAccruedLiabilities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOtherOperatingAssets" xlink:label="us-gaap_IncreaseDecreaseInOtherOperatingAssets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestPaidNet" xlink:label="us-gaap_InterestPaidNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LifeInsuranceCorporateOrBankOwnedChangeInValue" xlink:label="us-gaap_LifeInsuranceCorporateOrBankOwnedChangeInValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LifeSettlementContractsFairValueMethodGainLoss" xlink:label="us-gaap_LifeSettlementContractsFairValueMethodGainLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperationsAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperationsAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncashInvestingAndFinancingItemsAbstract" xlink:label="us-gaap_NoncashInvestingAndFinancingItemsAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForProceedsFromLoansReceivable" xlink:label="us-gaap_PaymentsForProceedsFromLoansReceivable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForRepurchaseOfCommonStock" xlink:label="us-gaap_PaymentsForRepurchaseOfCommonStock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsOfDividendsCommonStock" xlink:label="us-gaap_PaymentsOfDividendsCommonStock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsOfDividendsPreferredStockAndPreferenceStock" xlink:label="us-gaap_PaymentsOfDividendsPreferredStockAndPreferenceStock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt" xlink:label="us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireEquitySecuritiesFvNi" xlink:label="us-gaap_PaymentsToAcquireEquitySecuritiesFvNi" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireHeldToMaturitySecurities" xlink:label="us-gaap_PaymentsToAcquireHeldToMaturitySecurities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireLifeInsurancePolicies" xlink:label="us-gaap_PaymentsToAcquireLifeInsurancePolicies" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireLoansHeldForInvestment" xlink:label="us-gaap_PaymentsToAcquireLoansHeldForInvestment" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireOtherInvestments" xlink:label="us-gaap_PaymentsToAcquireOtherInvestments" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireOtherProductiveAssets" xlink:label="us-gaap_PaymentsToAcquireOtherProductiveAssets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested" xlink:label="us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromInsuranceSettlementInvestingActivities" xlink:label="us-gaap_ProceedsFromInsuranceSettlementInvestingActivities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfLongTermDebt" xlink:label="us-gaap_ProceedsFromIssuanceOfLongTermDebt" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" xlink:label="us-gaap_ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromLifeInsurancePolicies" xlink:label="us-gaap_ProceedsFromLifeInsurancePolicies" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities" xlink:label="us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfHeldToMaturitySecurities" xlink:label="us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfHeldToMaturitySecurities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromPaymentsForInSecuritiesSoldUnderAgreementsToRepurchase" xlink:label="us-gaap_ProceedsFromPaymentsForInSecuritiesSoldUnderAgreementsToRepurchase" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt" xlink:label="us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromSaleOfEquitySecuritiesFvNi" xlink:label="us-gaap_ProceedsFromSaleOfEquitySecuritiesFvNi" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromSaleOfOtherProductiveAssets" xlink:label="us-gaap_ProceedsFromSaleOfOtherProductiveAssets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromSaleOfOtherRealEstate" xlink:label="us-gaap_ProceedsFromSaleOfOtherRealEstate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromShortTermDebt" xlink:label="us-gaap_ProceedsFromShortTermDebt" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromStockOptionsExercised" xlink:label="us-gaap_ProceedsFromStockOptionsExercised" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProvisionForLoanLeaseAndOtherLosses" xlink:label="us-gaap_ProvisionForLoanLeaseAndOtherLosses" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepaymentsOfFederalHomeLoanBankBorrowings" xlink:label="us-gaap_RepaymentsOfFederalHomeLoanBankBorrowings" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepaymentsOfSubordinatedDebt" xlink:label="us-gaap_RepaymentsOfSubordinatedDebt" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensation" xlink:label="us-gaap_ShareBasedCompensation" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SupplementalCashFlowInformationAbstract" xlink:label="us-gaap_SupplementalCashFlowInformationAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransferToOtherRealEstate" xlink:label="us-gaap_TransferToOtherRealEstate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_FinancialInstrumentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="istr_EquitySecuritiesHeldInCorrespondentBanksMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="istr_MarketableSecuritiesAndMutualFundsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperationsAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_NetIncomeLoss" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_DepreciationAndAmortization" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_ProvisionForLoanLeaseAndOtherLosses" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="istr_AmortizationOfPurchaseAccountingAdjustments" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="istr_ProvisionForOtherRealEstateOwned" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_DebtSecuritiesRealizedGainLoss" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_GainLossOnSaleOfPropertyPlantEquipment" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="istr_GainLossOnSalesOfForeclosedAssets" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_GainLossOnSalesOfLoansNet" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_GainsLossesOnExtinguishmentOfDebt" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="istr_FederalHomeLoanBankStockDividends" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_ShareBasedCompensation" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_DeferredIncomeTaxExpenseBenefit" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_LifeInsuranceCorporateOrBankOwnedChangeInValue" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_LifeSettlementContractsFairValueMethodGainLoss" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_AmortizationOfFinancingCosts" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInAccruedInterestReceivableNet" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInOtherOperatingAssets" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInOtherAccruedLiabilities" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquireHeldToMaturitySecurities" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfHeldToMaturitySecurities" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromSaleOfEquitySecuritiesFvNi" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquireEquitySecuritiesFvNi" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsForProceedsFromLoansReceivable" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromSaleOfOtherRealEstate" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromSaleOfOtherProductiveAssets" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquireLoansHeldForInvestment" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquireOtherProductiveAssets" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquireLifeInsurancePolicies" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromLifeInsurancePolicies" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromInsuranceSettlementInvestingActivities" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquireOtherInvestments" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="istr_DistributionsFromInvestments" xlink:type="arc"/>
    <link:definitionArc order="17" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested" xlink:type="arc"/>
    <link:definitionArc order="18" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInInvestingActivities" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_IncreaseDecreaseInDeposits" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_ProceedsFromPaymentsForInSecuritiesSoldUnderAgreementsToRepurchase" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="istr_IncreaseDecreaseInShorttermFhlbAdvances" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_ProceedsFromShortTermDebt" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_ProceedsFromIssuanceOfLongTermDebt" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_RepaymentsOfFederalHomeLoanBankBorrowings" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_PaymentsOfDividendsCommonStock" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_PaymentsForRepurchaseOfCommonStock" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_ProceedsFromStockOptionsExercised" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_PaymentsOfDividendsPreferredStockAndPreferenceStock" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_RepaymentsOfSubordinatedDebt" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInFinancingActivities" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SupplementalCashFlowInformationAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SupplementalCashFlowInformationAbstract" xlink:to="us-gaap_InterestPaidNet" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SupplementalCashFlowInformationAbstract" xlink:to="us-gaap_IncomeTaxesPaid" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SupplementalCashFlowInformationAbstract" xlink:to="us-gaap_IncomeTaxPaidFederalBeforeRefundReceived" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SupplementalCashFlowInformationAbstract" xlink:to="us-gaap_IncomeTaxPaidStateAndLocalBeforeRefundReceived" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NoncashInvestingAndFinancingItemsAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoncashInvestingAndFinancingItemsAbstract" xlink:to="us-gaap_TransferToOtherRealEstate" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NoncashInvestingAndFinancingItemsAbstract" xlink:to="istr_TransferFromBankPremisesAndEquipmentToOtherAssets" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-consolidated-statements-of-comprehensive-income" xlink:href="istr-20251231.xsd#statement-consolidated-statements-of-comprehensive-income" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-consolidated-statements-of-comprehensive-income" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComprehensiveIncomeNetOfTax" xlink:label="us-gaap_ComprehensiveIncomeNetOfTax" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeAvailableForSaleSecuritiesAdjustmentNetOfTaxPeriodIncreaseDecreaseAbstract" xlink:label="us-gaap_OtherComprehensiveIncomeAvailableForSaleSecuritiesAdjustmentNetOfTaxPeriodIncreaseDecreaseAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" xlink:label="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax" xlink:label="us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax" xlink:label="us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetIncomeLoss" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherComprehensiveIncomeAvailableForSaleSecuritiesAdjustmentNetOfTaxPeriodIncreaseDecreaseAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherComprehensiveIncomeAvailableForSaleSecuritiesAdjustmentNetOfTaxPeriodIncreaseDecreaseAbstract" xlink:to="us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherComprehensiveIncomeAvailableForSaleSecuritiesAdjustmentNetOfTaxPeriodIncreaseDecreaseAbstract" xlink:to="us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherComprehensiveIncomeAvailableForSaleSecuritiesAdjustmentNetOfTaxPeriodIncreaseDecreaseAbstract" xlink:to="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherComprehensiveIncomeAvailableForSaleSecuritiesAdjustmentNetOfTaxPeriodIncreaseDecreaseAbstract" xlink:to="us-gaap_ComprehensiveIncomeNetOfTax" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-sales-of-investments-classified-as-afs-details" xlink:href="istr-20251231.xsd#statement-note-2-investment-securities-sales-of-investments-classified-as-afs-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-sales-of-investments-classified-as-afs-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleRealizedGain" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleRealizedGain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleRealizedLoss" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleRealizedLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt" xlink:label="us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtSecuritiesAvailableForSaleTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleRealizedGain" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleRealizedLoss" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-summary-of-continuous-unrealized-loss-position-for-securities-classified-as-afs-details" xlink:href="istr-20251231.xsd#statement-note-2-investment-securities-summary-of-continuous-unrealized-loss-position-for-securities-classified-as-afs-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-summary-of-continuous-unrealized-loss-position-for-securities-classified-as-afs-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialMortgageBackedSecuritiesMember" xlink:label="us-gaap_CommercialMortgageBackedSecuritiesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateDebtSecuritiesMember" xlink:label="us-gaap_CorporateDebtSecuritiesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionFairValueTable" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionFairValueTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResidentialMortgageBackedSecuritiesMember" xlink:label="us-gaap_ResidentialMortgageBackedSecuritiesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember" xlink:label="us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:label="us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionFairValueTable" xlink:to="us-gaap_FinancialInstrumentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_CorporateDebtSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_ResidentialMortgageBackedSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_CommercialMortgageBackedSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionFairValueTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionFairValueTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-summary-of-continuous-unrealized-loss-position-for-securities-classified-as-htm-details" xlink:href="istr-20251231.xsd#statement-note-2-investment-securities-summary-of-continuous-unrealized-loss-position-for-securities-classified-as-htm-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-summary-of-continuous-unrealized-loss-position-for-securities-classified-as-htm-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DebtSecuritiesHeldtomaturityContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss1" xlink:label="istr_DebtSecuritiesHeldtomaturityContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss1" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DebtSecuritiesHeldtomaturityContinuousUnrealizedLossPosition12MonthsOrLongerFairValue1" xlink:label="istr_DebtSecuritiesHeldtomaturityContinuousUnrealizedLossPosition12MonthsOrLongerFairValue1" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DebtSecuritiesHeldtomaturityContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss1" xlink:label="istr_DebtSecuritiesHeldtomaturityContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss1" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DebtSecuritiesHeldtomaturityContinuousUnrealizedLossPositionLessThan12MonthsFairValue1" xlink:label="istr_DebtSecuritiesHeldtomaturityContinuousUnrealizedLossPositionLessThan12MonthsFairValue1" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DebtSecuritiesHeldtomaturityUnrealizedLossPositionAccumulatedLoss1" xlink:label="istr_DebtSecuritiesHeldtomaturityUnrealizedLossPositionAccumulatedLoss1" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DebtSecuritiesHeldtomaturityUnrealizedLossPositionFairValue1" xlink:label="istr_DebtSecuritiesHeldtomaturityUnrealizedLossPositionFairValue1" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResidentialMortgageBackedSecuritiesMember" xlink:label="us-gaap_ResidentialMortgageBackedSecuritiesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfHeldToMaturitySecuritiesTable" xlink:label="us-gaap_ScheduleOfHeldToMaturitySecuritiesTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:label="us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfHeldToMaturitySecuritiesTable" xlink:to="us-gaap_FinancialInstrumentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_ResidentialMortgageBackedSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfHeldToMaturitySecuritiesTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfHeldToMaturitySecuritiesTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DebtSecuritiesHeldtomaturityContinuousUnrealizedLossPositionLessThan12MonthsFairValue1" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DebtSecuritiesHeldtomaturityContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss1" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DebtSecuritiesHeldtomaturityContinuousUnrealizedLossPosition12MonthsOrLongerFairValue1" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DebtSecuritiesHeldtomaturityContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss1" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DebtSecuritiesHeldtomaturityUnrealizedLossPositionFairValue1" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DebtSecuritiesHeldtomaturityUnrealizedLossPositionAccumulatedLoss1" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-contractual-maturity-details" xlink:href="istr-20251231.xsd#statement-note-2-investment-securities-contractual-maturity-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-2-investment-securities-contractual-maturity-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost" xlink:label="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue" xlink:label="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost" xlink:label="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue" xlink:label="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost" xlink:label="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue" xlink:label="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost" xlink:label="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue" xlink:label="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateAfterYear10" xlink:label="us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateAfterYear10" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateAfterYear5Through10" xlink:label="us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateAfterYear5Through10" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateAfterYearOneThroughFive" xlink:label="us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateAfterYearOneThroughFive" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateYearOne" xlink:label="us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateYearOne" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestAfterAllowanceForCreditLoss" xlink:label="us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestAfterAllowanceForCreditLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesHeldToMaturityPastDueTable" xlink:label="us-gaap_DebtSecuritiesHeldToMaturityPastDueTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue" xlink:label="us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue" xlink:label="us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue" xlink:label="us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue" xlink:label="us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HeldToMaturitySecuritiesFairValue" xlink:label="us-gaap_HeldToMaturitySecuritiesFairValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtSecuritiesHeldToMaturityPastDueTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesHeldToMaturityPastDueTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateYearOne" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateAfterYearOneThroughFive" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateAfterYear5Through10" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateAfterYear10" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue" xlink:type="arc"/>
    <link:definitionArc order="17" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="arc"/>
    <link:definitionArc order="18" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:type="arc"/>
    <link:definitionArc order="19" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestAfterAllowanceForCreditLoss" xlink:type="arc"/>
    <link:definitionArc order="20" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_HeldToMaturitySecuritiesFairValue" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-summary-of-loans-by-categories-details" xlink:href="istr-20251231.xsd#statement-note-3-loans-and-allowance-for-credit-losses-summary-of-loans-by-categories-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-summary-of-loans-by-categories-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_CommercialRealEstateLoanMember" xlink:label="istr_CommercialRealEstateLoanMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_FarmlandMember" xlink:label="istr_FarmlandMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_MultifamilyLoansMember" xlink:label="istr_MultifamilyLoansMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OneFourFamilyMember" xlink:label="istr_OneFourFamilyMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_RealEstatePortfolioSegmentMember" xlink:label="istr_RealEstatePortfolioSegmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialPortfolioSegmentMember" xlink:label="us-gaap_CommercialPortfolioSegmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConstructionLoansMember" xlink:label="us-gaap_ConstructionLoansMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsumerPortfolioSegmentMember" xlink:label="us-gaap_ConsumerPortfolioSegmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:label="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:label="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:label="us-gaap_FinancingReceivablePortfolioSegmentDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable" xlink:label="us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:to="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable" xlink:to="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:to="istr_RealEstatePortfolioSegmentMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:to="us-gaap_FinancingReceivablePortfolioSegmentDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="us-gaap_ConstructionLoansMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_OneFourFamilyMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_MultifamilyLoansMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_FarmlandMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_CommercialRealEstateLoanMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:to="us-gaap_CommercialPortfolioSegmentMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:to="us-gaap_ConsumerPortfolioSegmentMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-aging-analysis-of-loans-details" xlink:href="istr-20251231.xsd#statement-note-3-loans-and-allowance-for-credit-losses-aging-analysis-of-loans-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-aging-analysis-of-loans-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_FarmlandMember" xlink:label="istr_FarmlandMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_GreaterThanNinetyDaysAndAccruingMember" xlink:label="istr_GreaterThanNinetyDaysAndAccruingMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_MultifamilyLoansMember" xlink:label="istr_MultifamilyLoansMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OneFourFamilyMember" xlink:label="istr_OneFourFamilyMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_RealEstatePortfolioSegmentMember" xlink:label="istr_RealEstatePortfolioSegmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialPortfolioSegmentMember" xlink:label="us-gaap_CommercialPortfolioSegmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember" xlink:label="us-gaap_CommercialRealEstateMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConstructionLoansMember" xlink:label="us-gaap_ConstructionLoansMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsumerPortfolioSegmentMember" xlink:label="us-gaap_ConsumerPortfolioSegmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialAssetNotPastDueMember" xlink:label="us-gaap_FinancialAssetNotPastDueMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:label="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:label="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:label="us-gaap_FinancingReceivablePortfolioSegmentDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivables30To59DaysPastDueMember" xlink:label="us-gaap_FinancingReceivables30To59DaysPastDueMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivables60To89DaysPastDueMember" xlink:label="us-gaap_FinancingReceivables60To89DaysPastDueMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember" xlink:label="us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablesPeriodPastDueAxis" xlink:label="us-gaap_FinancingReceivablesPeriodPastDueAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablesPeriodPastDueDomain" xlink:label="us-gaap_FinancingReceivablesPeriodPastDueDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablesPeriodPastDueDomain" xlink:label="us-gaap_FinancingReceivablesPeriodPastDueDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable" xlink:label="us-gaap_ScheduleOfFinancingReceivablesPastDueTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:to="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancingReceivablesPeriodPastDueAxis" xlink:to="us-gaap_FinancingReceivablesPeriodPastDueDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFinancingReceivablesPastDueTable" xlink:to="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:to="istr_RealEstatePortfolioSegmentMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:to="us-gaap_FinancingReceivablePortfolioSegmentDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFinancingReceivablesPastDueTable" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="us-gaap_ConstructionLoansMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFinancingReceivablesPastDueTable" xlink:to="us-gaap_FinancingReceivablesPeriodPastDueAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablesPeriodPastDueDomain" xlink:to="us-gaap_FinancialAssetNotPastDueMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancingReceivablesPeriodPastDueAxis" xlink:to="us-gaap_FinancingReceivablesPeriodPastDueDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablesPeriodPastDueDomain" xlink:to="us-gaap_FinancingReceivables30To59DaysPastDueMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablesPeriodPastDueDomain" xlink:to="us-gaap_FinancingReceivables60To89DaysPastDueMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablesPeriodPastDueDomain" xlink:to="us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablesPeriodPastDueDomain" xlink:to="istr_GreaterThanNinetyDaysAndAccruingMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_OneFourFamilyMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_MultifamilyLoansMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_FarmlandMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="us-gaap_CommercialRealEstateMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:to="us-gaap_CommercialPortfolioSegmentMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:to="us-gaap_ConsumerPortfolioSegmentMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFinancingReceivablesPastDueTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfFinancingReceivablesPastDueTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-schedule-of-nonaccrual-loans-details" xlink:href="istr-20251231.xsd#statement-note-3-loans-and-allowance-for-credit-losses-schedule-of-nonaccrual-loans-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-schedule-of-nonaccrual-loans-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_FarmlandMember" xlink:label="istr_FarmlandMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_FinancingReceivableNonaccrualWithAllowance" xlink:label="istr_FinancingReceivableNonaccrualWithAllowance" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_MultifamilyLoansMember" xlink:label="istr_MultifamilyLoansMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OneFourFamilyMember" xlink:label="istr_OneFourFamilyMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_RealEstatePortfolioSegmentMember" xlink:label="istr_RealEstatePortfolioSegmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialPortfolioSegmentMember" xlink:label="us-gaap_CommercialPortfolioSegmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialRealEstateMember" xlink:label="us-gaap_CommercialRealEstateMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConstructionLoansMember" xlink:label="us-gaap_ConstructionLoansMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsumerPortfolioSegmentMember" xlink:label="us-gaap_ConsumerPortfolioSegmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrual" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrual" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrualNoAllowance" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrualNoAllowance" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableNonaccrualTable" xlink:label="us-gaap_FinancingReceivableNonaccrualTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:label="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:label="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:label="us-gaap_FinancingReceivablePortfolioSegmentDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:to="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FinancingReceivableNonaccrualTable" xlink:to="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:to="istr_RealEstatePortfolioSegmentMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:to="us-gaap_FinancingReceivablePortfolioSegmentDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FinancingReceivableNonaccrualTable" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="us-gaap_ConstructionLoansMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_OneFourFamilyMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_MultifamilyLoansMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_FarmlandMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="us-gaap_CommercialRealEstateMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:to="us-gaap_CommercialPortfolioSegmentMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:to="us-gaap_ConsumerPortfolioSegmentMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FinancingReceivableNonaccrualTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableNonaccrualTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrualNoAllowance" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_FinancingReceivableNonaccrualWithAllowance" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrual" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-loans-to-related-parties-details" xlink:href="istr-20251231.xsd#statement-note-3-loans-and-allowance-for-credit-losses-loans-to-related-parties-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-loans-to-related-parties-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoansAndLeasesReceivableRelatedParties" xlink:label="us-gaap_LoansAndLeasesReceivableRelatedParties" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoansAndLeasesReceivableRelatedPartiesAdditions" xlink:label="us-gaap_LoansAndLeasesReceivableRelatedPartiesAdditions" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoansAndLeasesReceivableRelatedPartiesProceeds" xlink:label="us-gaap_LoansAndLeasesReceivableRelatedPartiesProceeds" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LoansAndLeasesReceivableRelatedParties" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LoansAndLeasesReceivableRelatedPartiesAdditions" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LoansAndLeasesReceivableRelatedPartiesProceeds" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-allowance-for-loan-losses-details" xlink:href="istr-20251231.xsd#statement-note-3-loans-and-allowance-for-credit-losses-allowance-for-loan-losses-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-allowance-for-loan-losses-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_CommercialRealEstateLoanMember" xlink:label="istr_CommercialRealEstateLoanMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_FarmlandMember" xlink:label="istr_FarmlandMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_MultifamilyLoansMember" xlink:label="istr_MultifamilyLoansMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OneFourFamilyMember" xlink:label="istr_OneFourFamilyMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_RealEstatePortfolioSegmentMember" xlink:label="istr_RealEstatePortfolioSegmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember" xlink:label="srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionAxis" xlink:label="srt_CumulativeEffectPeriodOfAdoptionAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain" xlink:label="srt_CumulativeEffectPeriodOfAdoptionDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain" xlink:label="srt_CumulativeEffectPeriodOfAdoptionDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountingStandardsUpdate201613Member" xlink:label="us-gaap_AccountingStandardsUpdate201613Member" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:label="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialPortfolioSegmentMember" xlink:label="us-gaap_CommercialPortfolioSegmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConstructionLoansMember" xlink:label="us-gaap_ConstructionLoansMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsumerPortfolioSegmentMember" xlink:label="us-gaap_ConsumerPortfolioSegmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest" xlink:label="us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment" xlink:label="us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1" xlink:label="us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment" xlink:label="us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRecovery" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRecovery" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoff" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoff" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestCreditLossExpenseReversal" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestCreditLossExpenseReversal" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment" xlink:label="us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:label="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:label="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:label="us-gaap_FinancingReceivablePortfolioSegmentDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:label="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="us-gaap_TypeOfAdoptionMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="us-gaap_TypeOfAdoptionMember-default" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:to="us-gaap_TypeOfAdoptionMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:to="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_CumulativeEffectPeriodOfAdoptionAxis" xlink:to="srt_CumulativeEffectPeriodOfAdoptionDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="us-gaap_ConstructionLoansMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:to="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:to="istr_RealEstatePortfolioSegmentMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:to="us-gaap_FinancingReceivablePortfolioSegmentDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_OneFourFamilyMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_MultifamilyLoansMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_FarmlandMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_CommercialRealEstateLoanMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:to="us-gaap_CommercialPortfolioSegmentMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:to="us-gaap_ConsumerPortfolioSegmentMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:to="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TypeOfAdoptionMember" xlink:to="us-gaap_AccountingStandardsUpdate201613Member" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:to="us-gaap_TypeOfAdoptionMember-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:to="srt_CumulativeEffectPeriodOfAdoptionAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_CumulativeEffectPeriodOfAdoptionDomain" xlink:to="srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_CumulativeEffectPeriodOfAdoptionAxis" xlink:to="srt_CumulativeEffectPeriodOfAdoptionDomain-default" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestCreditLossExpenseReversal" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoff" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRecovery" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-4-other-real-estate-owned-real-estate-owned-details" xlink:href="istr-20251231.xsd#statement-note-4-other-real-estate-owned-real-estate-owned-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-4-other-real-estate-owned-real-estate-owned-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OtherRealEstateOwnedTransfersFromBankPremisesAndEquipment" xlink:label="istr_OtherRealEstateOwnedTransfersFromBankPremisesAndEquipment" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OtherRealEstateOwnedWriteDowns" xlink:label="istr_OtherRealEstateOwnedWriteDowns" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherRealEstate" xlink:label="us-gaap_OtherRealEstate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherRealEstateAdditions" xlink:label="us-gaap_OtherRealEstateAdditions" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherRealEstateDisposals" xlink:label="us-gaap_OtherRealEstateDisposals" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:label="us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TransferOfFinancialAssetsAccountedForAsSalesTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherRealEstate" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherRealEstateAdditions" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_OtherRealEstateOwnedTransfersFromBankPremisesAndEquipment" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherRealEstateDisposals" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_OtherRealEstateOwnedWriteDowns" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-5-bank-premises-and-equipment-bank-premises-and-equipment-details" xlink:href="istr-20251231.xsd#statement-note-5-bank-premises-and-equipment-bank-premises-and-equipment-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-5-bank-premises-and-equipment-bank-premises-and-equipment-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BuildingAndBuildingImprovementsMember" xlink:label="us-gaap_BuildingAndBuildingImprovementsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConstructionInProgressMember" xlink:label="us-gaap_ConstructionInProgressMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization" xlink:label="us-gaap_FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="us-gaap_FurnitureAndFixturesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LandMember" xlink:label="us-gaap_LandMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization" xlink:label="us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization" xlink:label="us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="us-gaap_PropertyPlantAndEquipmentGross" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SoftwareAndSoftwareDevelopmentCostsMember" xlink:label="us-gaap_SoftwareAndSoftwareDevelopmentCostsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_LandMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_BuildingAndBuildingImprovementsMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_FurnitureAndFixturesMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_SoftwareAndSoftwareDevelopmentCostsMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_ConstructionInProgressMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentGross" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-6-leases-quantitative-information-regarding-operating-leases-details" xlink:href="istr-20251231.xsd#statement-note-6-leases-quantitative-information-regarding-operating-leases-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-6-leases-quantitative-information-regarding-operating-leases-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeLeaseDescriptionTable" xlink:label="us-gaap_LesseeLeaseDescriptionTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseCost" xlink:label="us-gaap_OperatingLeaseCost" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent" xlink:label="us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1" xlink:label="us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_LesseeLeaseDescriptionTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LesseeLeaseDescriptionTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeaseCost" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-7-goodwill-and-other-intangible-assets-core-deposit-intangible-assets-details" xlink:href="istr-20251231.xsd#statement-note-7-goodwill-and-other-intangible-assets-core-deposit-intangible-assets-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-7-goodwill-and-other-intangible-assets-core-deposit-intangible-assets-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsGross" xlink:label="us-gaap_FiniteLivedIntangibleAssetsGross" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsNet" xlink:label="us-gaap_FiniteLivedIntangibleAssetsNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:label="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsGross" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsNet" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-8-deposits-deposits-details" xlink:href="istr-20251231.xsd#statement-note-8-deposits-deposits-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-8-deposits-deposits-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_InterestbearingDomesticDepositBrokeredDemandDeposits" xlink:label="istr_InterestbearingDomesticDepositBrokeredDemandDeposits" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_InterestbearingDomesticDepositBrokeredTimeDeposits" xlink:label="istr_InterestbearingDomesticDepositBrokeredTimeDeposits" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Deposits" xlink:label="us-gaap_Deposits" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestBearingDomesticDepositDemand" xlink:label="us-gaap_InterestBearingDomesticDepositDemand" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestBearingDomesticDepositMoneyMarket" xlink:label="us-gaap_InterestBearingDomesticDepositMoneyMarket" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestBearingDomesticDepositSavings" xlink:label="us-gaap_InterestBearingDomesticDepositSavings" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestBearingDomesticDepositTimeDeposits" xlink:label="us-gaap_InterestBearingDomesticDepositTimeDeposits" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoninterestBearingDepositLiabilities" xlink:label="us-gaap_NoninterestBearingDepositLiabilities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShortTermDebtTable" xlink:label="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShortTermDebtTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfShortTermDebtTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NoninterestBearingDepositLiabilities" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_InterestBearingDomesticDepositDemand" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_InterestBearingDomesticDepositMoneyMarket" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_InterestbearingDomesticDepositBrokeredDemandDeposits" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_InterestBearingDomesticDepositSavings" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_InterestbearingDomesticDepositBrokeredTimeDeposits" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_InterestBearingDomesticDepositTimeDeposits" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_Deposits" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes-income-tax-expense-details" xlink:href="istr-20251231.xsd#statement-note-16-income-taxes-income-tax-expense-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes-income-tax-expense-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CurrentFederalTaxExpenseBenefit" xlink:label="us-gaap_CurrentFederalTaxExpenseBenefit" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CurrentStateAndLocalTaxExpenseBenefit" xlink:label="us-gaap_CurrentStateAndLocalTaxExpenseBenefit" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredFederalIncomeTaxExpenseBenefit" xlink:label="us-gaap_DeferredFederalIncomeTaxExpenseBenefit" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:label="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="us-gaap_IncomeTaxExpenseBenefit" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CurrentFederalTaxExpenseBenefit" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CurrentStateAndLocalTaxExpenseBenefit" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredFederalIncomeTaxExpenseBenefit" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxExpenseBenefit" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments-quantitative-information-about-significant-unobservable-inputs-used-in-fair-value-measurement-details" xlink:href="istr-20251231.xsd#statement-note-17-fair-values-of-financial-instruments-quantitative-information-about-significant-unobservable-inputs-used-in-fair-value-measurement-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments-quantitative-information-about-significant-unobservable-inputs-used-in-fair-value-measurement-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_ImpairedFinancingReceivableMeasurementInput" xlink:label="istr_ImpairedFinancingReceivableMeasurementInput" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_MeasurementInputCollateralDiscountsAndDiscountRatesMember" xlink:label="istr_MeasurementInputCollateralDiscountsAndDiscountRatesMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_MeasurementInputCollateralDiscountsAndEstimatedCostsToSellMember" xlink:label="istr_MeasurementInputCollateralDiscountsAndEstimatedCostsToSellMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OtherRealEstateOwnedFairValueDisclosure" xlink:label="istr_OtherRealEstateOwnedFairValueDisclosure" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_WeightedAverageMember" xlink:label="srt_WeightedAverageMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateDebtSecuritiesMember" xlink:label="us-gaap_CorporateDebtSecuritiesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleMeasurementInput" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleMeasurementInput" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember" xlink:label="us-gaap_EstimateOfFairValueFairValueDisclosureMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementBasisAxis" xlink:label="us-gaap_FairValueByMeasurementBasisAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:label="us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment" xlink:label="us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputAppraisedValueMember" xlink:label="us-gaap_MeasurementInputAppraisedValueMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeAxis" xlink:label="us-gaap_MeasurementInputTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeDomain" xlink:label="us-gaap_MeasurementInputTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeDomain" xlink:label="us-gaap_MeasurementInputTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherRealEstateOwnedMeasurementInput" xlink:label="us-gaap_OtherRealEstateOwnedMeasurementInput" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:label="us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:label="us-gaap_PortionAtFairValueFairValueDisclosureMember-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:label="us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationTechniqueAxis" xlink:label="us-gaap_ValuationTechniqueAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationTechniqueDiscountedCashFlowMember" xlink:label="us-gaap_ValuationTechniqueDiscountedCashFlowMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationTechniqueDomain" xlink:label="us-gaap_ValuationTechniqueDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationTechniqueDomain" xlink:label="us-gaap_ValuationTechniqueDomain-default" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_MeasurementInputTypeAxis" xlink:to="us-gaap_MeasurementInputTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ValuationTechniqueAxis" xlink:to="us-gaap_ValuationTechniqueDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByMeasurementBasisAxis" xlink:to="us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:to="us-gaap_ValuationTechniqueAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ValuationTechniqueDomain" xlink:to="us-gaap_ValuationTechniqueDiscountedCashFlowMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ValuationTechniqueAxis" xlink:to="us-gaap_ValuationTechniqueDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:to="us-gaap_MeasurementInputTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_MeasurementInputTypeDomain" xlink:to="us-gaap_MeasurementInputAppraisedValueMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_MeasurementInputTypeAxis" xlink:to="us-gaap_MeasurementInputTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:to="us-gaap_FinancialInstrumentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:to="srt_RangeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_WeightedAverageMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_MeasurementInputTypeDomain" xlink:to="istr_MeasurementInputCollateralDiscountsAndEstimatedCostsToSellMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:to="us-gaap_FairValueByMeasurementBasisAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:to="us-gaap_EstimateOfFairValueFairValueDisclosureMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByMeasurementBasisAxis" xlink:to="us-gaap_PortionAtFairValueFairValueDisclosureMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_CorporateDebtSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_MeasurementInputTypeDomain" xlink:to="istr_MeasurementInputCollateralDiscountsAndDiscountRatesMember" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleMeasurementInput" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_ImpairedFinancingReceivableMeasurementInput" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_OtherRealEstateOwnedFairValueDisclosure" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherRealEstateOwnedMeasurementInput" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-22-earnings-per-common-share-earnings-per-share-details" xlink:href="istr-20251231.xsd#statement-note-22-earnings-per-common-share-earnings-per-share-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-22-earnings-per-common-share-earnings-per-share-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_NetIncomeLossAllocatedToCommonStockholdersBasic" xlink:label="istr_NetIncomeLossAllocatedToCommonStockholdersBasic" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasic" xlink:label="us-gaap_EarningsPerShareBasic" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDiluted" xlink:label="us-gaap_EarningsPerShareDiluted" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncrementalCommonSharesAttributableToConversionOfPreferredStock" xlink:label="us-gaap_IncrementalCommonSharesAttributableToConversionOfPreferredStock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements" xlink:label="us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic" xlink:label="us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ParticipatingSecuritiesDistributedAndUndistributedEarningsLossBasic" xlink:label="us-gaap_ParticipatingSecuritiesDistributedAndUndistributedEarningsLossBasic" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockDividendsIncomeStatementImpact" xlink:label="us-gaap_PreferredStockDividendsIncomeStatementImpact" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable" xlink:label="us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:label="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" xlink:label="us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetIncomeLoss" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockDividendsIncomeStatementImpact" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ParticipatingSecuritiesDistributedAndUndistributedEarningsLossBasic" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_NetIncomeLossAllocatedToCommonStockholdersBasic" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncrementalCommonSharesAttributableToConversionOfPreferredStock" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EarningsPerShareBasic" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EarningsPerShareDiluted" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-22-earnings-per-common-share-antidilutive-securities-details" xlink:href="istr-20251231.xsd#statement-note-22-earnings-per-common-share-antidilutive-securities-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-22-earnings-per-common-share-antidilutive-securities-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="us-gaap_EmployeeStockOptionMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="us-gaap_RestrictedStockUnitsRSUMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:to="us-gaap_AwardTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_EmployeeStockOptionMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_RestrictedStockUnitsRSUMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-21-parent-company-only-financial-statements-balance-sheets-details" xlink:href="istr-20251231.xsd#statement-note-21-parent-company-only-financial-statements-balance-sheets-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-21-parent-company-only-financial-statements-balance-sheets-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DueFromBankSubsidiary" xlink:label="istr_DueFromBankSubsidiary" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_InvestmentInTrust" xlink:label="istr_InvestmentInTrust" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesAxis" xlink:label="srt_ConsolidatedEntitiesAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain" xlink:label="srt_ConsolidatedEntitiesDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain" xlink:label="srt_ConsolidatedEntitiesDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ParentCompanyMember" xlink:label="srt_ParentCompanyMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfCondensedFinancialStatementsTable" xlink:label="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableCurrentAndNoncurrent" xlink:label="us-gaap_AccountsPayableCurrentAndNoncurrent" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalCommonStock" xlink:label="us-gaap_AdditionalPaidInCapitalCommonStock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Assets" xlink:label="us-gaap_Assets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract" xlink:label="us-gaap_AssetsAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndDueFromBanks" xlink:label="us-gaap_CashAndDueFromBanks" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockValue" xlink:label="us-gaap_CommonStockValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxLiabilitiesNet" xlink:label="us-gaap_DeferredIncomeTaxLiabilitiesNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsPayableCurrentAndNoncurrent" xlink:label="us-gaap_DividendsPayableCurrentAndNoncurrent" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestments" xlink:label="us-gaap_EquityMethodInvestments" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiCost" xlink:label="us-gaap_EquitySecuritiesFvNiCost" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedTrademarks" xlink:label="us-gaap_IndefiniteLivedTrademarks" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestPayableCurrentAndNoncurrent" xlink:label="us-gaap_InterestPayableCurrentAndNoncurrent" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust" xlink:label="us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Liabilities" xlink:label="us-gaap_Liabilities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAbstract" xlink:label="us-gaap_LiabilitiesAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquity" xlink:label="us-gaap_LiabilitiesAndStockholdersEquity" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssets" xlink:label="us-gaap_OtherAssets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockValue" xlink:label="us-gaap_PreferredStockValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsAccumulatedDeficit" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity" xlink:label="us-gaap_StockholdersEquity" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityAbstract" xlink:label="us-gaap_StockholdersEquityAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedDebt" xlink:label="us-gaap_SubordinatedDebt" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_ConsolidatedEntitiesAxis" xlink:to="srt_ConsolidatedEntitiesDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:to="srt_ConsolidatedEntitiesAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_ConsolidatedEntitiesDomain" xlink:to="srt_ParentCompanyMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_ConsolidatedEntitiesAxis" xlink:to="srt_ConsolidatedEntitiesDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AssetsAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_CashAndDueFromBanks" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_EquitySecuritiesFvNiCost" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="istr_DueFromBankSubsidiary" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_EquityMethodInvestments" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="istr_InvestmentInTrust" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_IndefiniteLivedTrademarks" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_OtherAssets" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_Assets" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LiabilitiesAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAbstract" xlink:to="us-gaap_SubordinatedDebt" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAbstract" xlink:to="us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAbstract" xlink:to="us-gaap_AccountsPayableCurrentAndNoncurrent" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAbstract" xlink:to="us-gaap_InterestPayableCurrentAndNoncurrent" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAbstract" xlink:to="us-gaap_DividendsPayableCurrentAndNoncurrent" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAbstract" xlink:to="us-gaap_DeferredIncomeTaxLiabilitiesNet" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAbstract" xlink:to="us-gaap_Liabilities" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockholdersEquityAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_PreferredStockValue" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_CommonStockValue" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_AdditionalPaidInCapitalCommonStock" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_RetainedEarningsAccumulatedDeficit" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_StockholdersEquity" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_LiabilitiesAndStockholdersEquity" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-21-parent-company-only-financial-statements-statements-of-operations-details" xlink:href="istr-20251231.xsd#statement-note-21-parent-company-only-financial-statements-statements-of-operations-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-21-parent-company-only-financial-statements-statements-of-operations-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DividendIncomeFromSubsidiary" xlink:label="istr_DividendIncomeFromSubsidiary" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_IncomeLossBeforeEquityInUndistributedIncomeLossOfSubsidiaryIncomeTaxes" xlink:label="istr_IncomeLossBeforeEquityInUndistributedIncomeLossOfSubsidiaryIncomeTaxes" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_ManagementServiceFeesToSubsidiaryBank" xlink:label="istr_ManagementServiceFeesToSubsidiaryBank" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesAxis" xlink:label="srt_ConsolidatedEntitiesAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain" xlink:label="srt_ConsolidatedEntitiesDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain" xlink:label="srt_ConsolidatedEntitiesDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ParentCompanyMember" xlink:label="srt_ParentCompanyMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfCondensedFinancialStatementsTable" xlink:label="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts" xlink:label="us-gaap_BusinessCombinationAcquisitionRelatedCosts" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendIncomeOperating" xlink:label="us-gaap_DividendIncomeOperating" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss" xlink:label="us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt" xlink:label="us-gaap_GainsLossesOnExtinguishmentOfDebt" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromSubsidiariesBeforeTax" xlink:label="us-gaap_IncomeLossFromSubsidiariesBeforeTax" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="us-gaap_IncomeTaxExpenseBenefit" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseBorrowings" xlink:label="us-gaap_InterestExpenseBorrowings" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestIncomeOperating" xlink:label="us-gaap_InterestIncomeOperating" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic" xlink:label="us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingExpenses" xlink:label="us-gaap_OperatingExpenses" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingExpensesAbstract" xlink:label="us-gaap_OperatingExpensesAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherExpenses" xlink:label="us-gaap_OtherExpenses" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherOperatingIncome" xlink:label="us-gaap_OtherOperatingIncome" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockDividendsIncomeStatementImpact" xlink:label="us-gaap_PreferredStockDividendsIncomeStatementImpact" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Revenues" xlink:label="us-gaap_Revenues" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenuesAbstract" xlink:label="us-gaap_RevenuesAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_ConsolidatedEntitiesAxis" xlink:to="srt_ConsolidatedEntitiesDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:to="srt_ConsolidatedEntitiesAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_ConsolidatedEntitiesDomain" xlink:to="srt_ParentCompanyMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_ConsolidatedEntitiesAxis" xlink:to="srt_ConsolidatedEntitiesDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_RevenuesAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RevenuesAbstract" xlink:to="istr_DividendIncomeFromSubsidiary" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RevenuesAbstract" xlink:to="us-gaap_DividendIncomeOperating" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RevenuesAbstract" xlink:to="us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RevenuesAbstract" xlink:to="us-gaap_InterestIncomeOperating" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RevenuesAbstract" xlink:to="us-gaap_OtherOperatingIncome" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RevenuesAbstract" xlink:to="us-gaap_Revenues" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingExpensesAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_InterestExpenseBorrowings" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="istr_ManagementServiceFeesToSubsidiaryBank" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_GainsLossesOnExtinguishmentOfDebt" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_BusinessCombinationAcquisitionRelatedCosts" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_OtherExpenses" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_OperatingExpenses" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="istr_IncomeLossBeforeEquityInUndistributedIncomeLossOfSubsidiaryIncomeTaxes" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_IncomeLossFromSubsidiariesBeforeTax" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_IncomeTaxExpenseBenefit" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_NetIncomeLoss" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_PreferredStockDividendsIncomeStatementImpact" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-21-parent-company-only-financial-statements-statements-of-cash-flows-details" xlink:href="istr-20251231.xsd#statement-note-21-parent-company-only-financial-statements-statements-of-cash-flows-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-21-parent-company-only-financial-statements-statements-of-cash-flows-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_IncreaseDecreaseInDueFromBankSubsidiary" xlink:label="istr_IncreaseDecreaseInDueFromBankSubsidiary" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesAxis" xlink:label="srt_ConsolidatedEntitiesAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain" xlink:label="srt_ConsolidatedEntitiesDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain" xlink:label="srt_ConsolidatedEntitiesDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ParentCompanyMember" xlink:label="srt_ParentCompanyMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfCondensedFinancialStatementsTable" xlink:label="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfFinancingCosts" xlink:label="us-gaap_AmortizationOfFinancingCosts" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss" xlink:label="us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt" xlink:label="us-gaap_GainsLossesOnExtinguishmentOfDebt" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromEquityMethodInvestments" xlink:label="us-gaap_IncomeLossFromEquityMethodInvestments" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInDeferredIncomeTaxes" xlink:label="us-gaap_IncreaseDecreaseInDeferredIncomeTaxes" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:label="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOtherAccruedLiabilities" xlink:label="us-gaap_IncreaseDecreaseInOtherAccruedLiabilities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOtherOperatingAssets" xlink:label="us-gaap_IncreaseDecreaseInOtherOperatingAssets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestPaidNet" xlink:label="us-gaap_InterestPaidNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForRepurchaseOfCommonStock" xlink:label="us-gaap_PaymentsForRepurchaseOfCommonStock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsOfDividendsCommonStock" xlink:label="us-gaap_PaymentsOfDividendsCommonStock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsOfDividendsPreferredStockAndPreferenceStock" xlink:label="us-gaap_PaymentsOfDividendsPreferredStockAndPreferenceStock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireAdditionalInterestInSubsidiaries" xlink:label="us-gaap_PaymentsToAcquireAdditionalInterestInSubsidiaries" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireEquitySecuritiesFvNi" xlink:label="us-gaap_PaymentsToAcquireEquitySecuritiesFvNi" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireOtherInvestments" xlink:label="us-gaap_PaymentsToAcquireOtherInvestments" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" xlink:label="us-gaap_ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromStockOptionsExercised" xlink:label="us-gaap_ProceedsFromStockOptionsExercised" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepaymentsOfSubordinatedDebt" xlink:label="us-gaap_RepaymentsOfSubordinatedDebt" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SupplementalCashFlowInformationAbstract" xlink:label="us-gaap_SupplementalCashFlowInformationAbstract" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_ConsolidatedEntitiesAxis" xlink:to="srt_ConsolidatedEntitiesDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:to="srt_ConsolidatedEntitiesAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_ConsolidatedEntitiesDomain" xlink:to="srt_ParentCompanyMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_ConsolidatedEntitiesAxis" xlink:to="srt_ConsolidatedEntitiesDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="srt_ScheduleOfCondensedFinancialStatementsTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_NetIncomeLoss" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_IncomeLossFromEquityMethodInvestments" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_AmortizationOfFinancingCosts" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_GainsLossesOnExtinguishmentOfDebt" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="istr_IncreaseDecreaseInDueFromBankSubsidiary" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInOtherOperatingAssets" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInDeferredIncomeTaxes" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInOtherAccruedLiabilities" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquireEquitySecuritiesFvNi" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquireOtherInvestments" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquireAdditionalInterestInSubsidiaries" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInInvestingActivities" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_PaymentsOfDividendsCommonStock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_PaymentsOfDividendsPreferredStockAndPreferenceStock" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_PaymentsForRepurchaseOfCommonStock" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_ProceedsFromStockOptionsExercised" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_RepaymentsOfSubordinatedDebt" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInFinancingActivities" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SupplementalCashFlowInformationAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SupplementalCashFlowInformationAbstract" xlink:to="us-gaap_InterestPaidNet" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-loan-portfolio-by-category-and-credit-quality-indicator-details" xlink:href="istr-20251231.xsd#statement-note-3-loans-and-allowance-for-credit-losses-loan-portfolio-by-category-and-credit-quality-indicator-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-3-loans-and-allowance-for-credit-losses-loan-portfolio-by-category-and-credit-quality-indicator-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_CommercialRealEstateLoanMember" xlink:label="istr_CommercialRealEstateLoanMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_FarmlandMember" xlink:label="istr_FarmlandMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_MultifamilyLoansMember" xlink:label="istr_MultifamilyLoansMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_OneFourFamilyMember" xlink:label="istr_OneFourFamilyMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_RealEstatePortfolioSegmentMember" xlink:label="istr_RealEstatePortfolioSegmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_FinancingReceivableCreditRatioTable" xlink:label="srt_FinancingReceivableCreditRatioTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialPortfolioSegmentMember" xlink:label="us-gaap_CommercialPortfolioSegmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConstructionLoansMember" xlink:label="us-gaap_ConstructionLoansMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsumerPortfolioSegmentMember" xlink:label="us-gaap_ConsumerPortfolioSegmentMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoff" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoff" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYear" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYear" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestRevolving" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestRevolving" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingWriteoff" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingWriteoff" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYear" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYear" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYear" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYear" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYear" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYear" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYearWriteoff" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYearWriteoff" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYear" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYear" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYear" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYear" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff" xlink:label="us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:label="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:label="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:label="us-gaap_FinancingReceivablePortfolioSegmentDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:label="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InternalCreditAssessmentAxis" xlink:label="us-gaap_InternalCreditAssessmentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InternalCreditAssessmentDomain" xlink:label="us-gaap_InternalCreditAssessmentDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InternalCreditAssessmentDomain" xlink:label="us-gaap_InternalCreditAssessmentDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PassMember" xlink:label="us-gaap_PassMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SpecialMentionMember" xlink:label="us-gaap_SpecialMentionMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubstandardMember" xlink:label="us-gaap_SubstandardMember" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:to="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_InternalCreditAssessmentAxis" xlink:to="us-gaap_InternalCreditAssessmentDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="srt_FinancingReceivableCreditRatioTable" xlink:to="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:to="istr_RealEstatePortfolioSegmentMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentAxis" xlink:to="us-gaap_FinancingReceivablePortfolioSegmentDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="srt_FinancingReceivableCreditRatioTable" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="us-gaap_ConstructionLoansMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis" xlink:to="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="srt_FinancingReceivableCreditRatioTable" xlink:to="us-gaap_InternalCreditAssessmentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InternalCreditAssessmentDomain" xlink:to="us-gaap_PassMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_InternalCreditAssessmentAxis" xlink:to="us-gaap_InternalCreditAssessmentDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InternalCreditAssessmentDomain" xlink:to="us-gaap_SpecialMentionMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InternalCreditAssessmentDomain" xlink:to="us-gaap_SubstandardMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_OneFourFamilyMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_MultifamilyLoansMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_FarmlandMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain" xlink:to="istr_CommercialRealEstateLoanMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:to="us-gaap_CommercialPortfolioSegmentMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FinancingReceivablePortfolioSegmentDomain" xlink:to="us-gaap_ConsumerPortfolioSegmentMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="srt_FinancingReceivableCreditRatioTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="srt_FinancingReceivableCreditRatioTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYear" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYear" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYear" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYear" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYear" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYear" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestRevolving" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYearWriteoff" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingWriteoff" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoff" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-11-other-borrowed-funds-federal-home-loan-bank-advances-details" xlink:href="istr-20251231.xsd#statement-note-11-other-borrowed-funds-federal-home-loan-bank-advances-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-11-other-borrowed-funds-federal-home-loan-bank-advances-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdvancesFromFederalHomeLoanBanks" xlink:label="us-gaap_AdvancesFromFederalHomeLoanBanks" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable" xlink:label="us-gaap_DebtInstrumentTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankInterestRate" xlink:label="us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankInterestRate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryAverageInterestRateOfAmountsDueWithinOneYearOfBalanceSheetDate" xlink:label="us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryAverageInterestRateOfAmountsDueWithinOneYearOfBalanceSheetDate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryAverageInterestRateOneToTwoYearsFromBalanceSheetDate" xlink:label="us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryAverageInterestRateOneToTwoYearsFromBalanceSheetDate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryAverageInterestRateTwoToThreeYearsFromBalanceSheetDate" xlink:label="us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryAverageInterestRateTwoToThreeYearsFromBalanceSheetDate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryDueFromOneToTwoYearsOfBalanceSheetDate" xlink:label="us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryDueFromOneToTwoYearsOfBalanceSheetDate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryDueFromTwoToThreeYearsOfBalanceSheetDate" xlink:label="us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryDueFromTwoToThreeYearsOfBalanceSheetDate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryDueWithinOneYearOfBalanceSheetDate" xlink:label="us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryDueWithinOneYearOfBalanceSheetDate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtInstrumentTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryDueWithinOneYearOfBalanceSheetDate" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryAverageInterestRateOfAmountsDueWithinOneYearOfBalanceSheetDate" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryDueFromOneToTwoYearsOfBalanceSheetDate" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryAverageInterestRateOneToTwoYearsFromBalanceSheetDate" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryDueFromTwoToThreeYearsOfBalanceSheetDate" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryAverageInterestRateTwoToThreeYearsFromBalanceSheetDate" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AdvancesFromFederalHomeLoanBanks" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankInterestRate" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-12-derivative-financial-instruments-schedule-of-derivatives-instruments-details" xlink:href="istr-20251231.xsd#statement-note-12-derivative-financial-instruments-schedule-of-derivatives-instruments-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-12-derivative-financial-instruments-schedule-of-derivatives-instruments-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeFairValueOfDerivativeAsset" xlink:label="us-gaap_DerivativeFairValueOfDerivativeAsset" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeFairValueOfDerivativeLiability" xlink:label="us-gaap_DerivativeFairValueOfDerivativeLiability" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresTable" xlink:label="us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeNotionalAmount" xlink:label="us-gaap_DerivativeNotionalAmount" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestRateSwapMember" xlink:label="us-gaap_InterestRateSwapMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresTable" xlink:to="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="us-gaap_InterestRateSwapMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeNotionalAmount" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeFairValueOfDerivativeAsset" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeFairValueOfDerivativeLiability" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-schedule-of-restricted-stock-activity-details" xlink:href="istr-20251231.xsd#statement-note-14-stockbased-compensation-schedule-of-restricted-stock-activity-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-14-stockbased-compensation-schedule-of-restricted-stock-activity-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_RestrictedStockAndRestrictedStockUnitsMember" xlink:label="istr_RestrictedStockAndRestrictedStockUnitsMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_AwardTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="istr_RestrictedStockAndRestrictedStockUnitsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes-summary-of-provision-for-federal-income-taxes-details" xlink:href="istr-20251231.xsd#statement-note-16-income-taxes-summary-of-provision-for-federal-income-taxes-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes-summary-of-provision-for-federal-income-taxes-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DomesticCountryMember" xlink:label="us-gaap_DomesticCountryMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperations" xlink:label="us-gaap_EffectiveIncomeTaxRateContinuingOperations" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:label="us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseLifeInsurance" xlink:label="us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseLifeInsurance" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther" xlink:label="us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments" xlink:label="us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes" xlink:label="us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:label="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTaxExemptIncome" xlink:label="us-gaap_EffectiveIncomeTaxRateReconciliationTaxExemptIncome" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:label="us-gaap_IncomeTaxAuthorityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="us-gaap_IncomeTaxAuthorityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="us-gaap_IncomeTaxAuthorityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="us-gaap_IncomeTaxExpenseBenefit" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate" xlink:label="us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationNondeductibleExpenseLifeInsurance" xlink:label="us-gaap_IncomeTaxReconciliationNondeductibleExpenseLifeInsurance" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther" xlink:label="us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationOtherAdjustments" xlink:label="us-gaap_IncomeTaxReconciliationOtherAdjustments" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes" xlink:label="us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationTaxExemptIncome" xlink:label="us-gaap_IncomeTaxReconciliationTaxExemptIncome" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_IncomeTaxAuthorityAxis" xlink:to="us-gaap_IncomeTaxAuthorityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:to="us-gaap_IncomeTaxAuthorityAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeTaxAuthorityDomain" xlink:to="us-gaap_DomesticCountryMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_IncomeTaxAuthorityAxis" xlink:to="us-gaap_IncomeTaxAuthorityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxReconciliationTaxExemptIncome" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EffectiveIncomeTaxRateReconciliationTaxExemptIncome" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxReconciliationNondeductibleExpenseLifeInsurance" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseLifeInsurance" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxReconciliationOtherAdjustments" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxExpenseBenefit" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EffectiveIncomeTaxRateContinuingOperations" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes-summary-of-deferred-tax-assets-and-liabilities-details" xlink:href="istr-20251231.xsd#statement-note-16-income-taxes-summary-of-deferred-tax-assets-and-liabilities-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-16-income-taxes-summary-of-deferred-tax-assets-and-liabilities-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DeferredTaxAssetsBasisDifferenceInAcquiredAssetsAndLiabilities" xlink:label="istr_DeferredTaxAssetsBasisDifferenceInAcquiredAssetsAndLiabilities" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DeferredTaxAssetsOperatingLeaseLiability" xlink:label="istr_DeferredTaxAssetsOperatingLeaseLiability" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DeferredTaxAssetsUnearnedLoanFees" xlink:label="istr_DeferredTaxAssetsUnearnedLoanFees" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DeferredTaxLiabilitiesBasisDifferenceInAcquiredAssetsAndLiabilities" xlink:label="istr_DeferredTaxLiabilitiesBasisDifferenceInAcquiredAssetsAndLiabilities" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DeferredTaxLiabilitiesFederalHomeLoanBankStockDividend" xlink:label="istr_DeferredTaxLiabilitiesFederalHomeLoanBankStockDividend" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxLiabilities" xlink:label="us-gaap_DeferredIncomeTaxLiabilities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsGross" xlink:label="us-gaap_DeferredTaxAssetsGross" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsLiabilitiesNet" xlink:label="us-gaap_DeferredTaxAssetsLiabilitiesNet" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsOther" xlink:label="us-gaap_DeferredTaxAssetsOther" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeCompensation" xlink:label="us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeCompensation" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost" xlink:label="us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsProvisionForLoanLosses" xlink:label="us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsProvisionForLoanLosses" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsUnrealizedLossesOnAvailableforSaleSecuritiesGross" xlink:label="us-gaap_DeferredTaxAssetsUnrealizedLossesOnAvailableforSaleSecuritiesGross" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxLiabilitiesLeasingArrangements" xlink:label="us-gaap_DeferredTaxLiabilitiesLeasingArrangements" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxLiabilitiesOther" xlink:label="us-gaap_DeferredTaxLiabilitiesOther" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment" xlink:label="us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:label="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DeferredTaxLiabilitiesFederalHomeLoanBankStockDividend" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DeferredTaxLiabilitiesBasisDifferenceInAcquiredAssetsAndLiabilities" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxLiabilitiesLeasingArrangements" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxLiabilitiesOther" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredIncomeTaxLiabilities" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsProvisionForLoanLosses" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxAssetsUnrealizedLossesOnAvailableforSaleSecuritiesGross" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeCompensation" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DeferredTaxAssetsBasisDifferenceInAcquiredAssetsAndLiabilities" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DeferredTaxAssetsOperatingLeaseLiability" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DeferredTaxAssetsUnearnedLoanFees" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxAssetsOther" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxAssetsGross" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxAssetsLiabilitiesNet" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments-estimated-fair-value-of-financial-instruments-details" xlink:href="istr-20251231.xsd#statement-note-17-fair-values-of-financial-instruments-estimated-fair-value-of-financial-instruments-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments-estimated-fair-value-of-financial-instruments-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_DebtSecuritiesHeldtomaturityFairValueDisclosure" xlink:label="istr_DebtSecuritiesHeldtomaturityFairValueDisclosure" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_EquitySecuritiesHeldInCorrespondentBanksMember" xlink:label="istr_EquitySecuritiesHeldInCorrespondentBanksMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_FederalHomeLoanBankLongTermBorrowingsFairValueDisclosure" xlink:label="istr_FederalHomeLoanBankLongTermBorrowingsFairValueDisclosure" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_InterestBearingDepositLiabilitiesFairValueDisclosure" xlink:label="istr_InterestBearingDepositLiabilitiesFairValueDisclosure" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_MarketableSecuritiesAndMutualFundsMember" xlink:label="istr_MarketableSecuritiesAndMutualFundsMember" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_NoninterestBearingDepositLiabilitiesFairValueDisclosure" xlink:label="istr_NoninterestBearingDepositLiabilitiesFairValueDisclosure" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember" xlink:label="us-gaap_CarryingReportedAmountFairValueDisclosureMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure" xlink:label="us-gaap_CashAndCashEquivalentsFairValueDisclosure" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepositsFairValueDisclosure" xlink:label="us-gaap_DepositsFairValueDisclosure" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral" xlink:label="us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral" xlink:label="us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent" xlink:label="us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember" xlink:label="us-gaap_EstimateOfFairValueFairValueDisclosureMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByBalanceSheetGroupingTable" xlink:label="us-gaap_FairValueByBalanceSheetGroupingTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementBasisAxis" xlink:label="us-gaap_FairValueByMeasurementBasisAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="us-gaap_FairValueInputsLevel1Member" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="us-gaap_FairValueInputsLevel2Member" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="us-gaap_FairValueInputsLevel3Member" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations" xlink:label="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestRateSwapMember" xlink:label="us-gaap_InterestRateSwapMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust" xlink:label="us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoansReceivableFairValueDisclosure" xlink:label="us-gaap_LoansReceivableFairValueDisclosure" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:label="us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:label="us-gaap_PortionAtFairValueFairValueDisclosureMember-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShorttermDebtFairValue" xlink:label="us-gaap_ShorttermDebtFairValue" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByMeasurementBasisAxis" xlink:to="us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueByBalanceSheetGroupingTable" xlink:to="us-gaap_FairValueByMeasurementBasisAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:to="us-gaap_CarryingReportedAmountFairValueDisclosureMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByMeasurementBasisAxis" xlink:to="us-gaap_PortionAtFairValueFairValueDisclosureMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:to="us-gaap_EstimateOfFairValueFairValueDisclosureMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueByBalanceSheetGroupingTable" xlink:to="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel1Member" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel2Member" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel3Member" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueByBalanceSheetGroupingTable" xlink:to="us-gaap_FinancialInstrumentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="istr_MarketableSecuritiesAndMutualFundsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="istr_EquitySecuritiesHeldInCorrespondentBanksMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueByBalanceSheetGroupingTable" xlink:to="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="us-gaap_InterestRateSwapMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueByBalanceSheetGroupingTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueByBalanceSheetGroupingTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CashAndCashEquivalentsFairValueDisclosure" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_DebtSecuritiesHeldtomaturityFairValueDisclosure" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LoansReceivableFairValueDisclosure" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DepositsFairValueDisclosure" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShorttermDebtFairValue" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_FederalHomeLoanBankLongTermBorrowingsFairValueDisclosure" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_NoninterestBearingDepositLiabilitiesFairValueDisclosure" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_InterestBearingDepositLiabilitiesFairValueDisclosure" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-12-derivative-financial-instruments-offsetting-of-derivative-instruments-details" xlink:href="istr-20251231.xsd#statement-note-12-derivative-financial-instruments-offsetting-of-derivative-instruments-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-12-derivative-financial-instruments-offsetting-of-derivative-instruments-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeAssetFairValueAfterOffsetAndDeductionSubjectToMasterNettingArrangement" xlink:label="us-gaap_DerivativeAssetFairValueAfterOffsetAndDeductionSubjectToMasterNettingArrangement" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeAssetFairValueGrossLiability" xlink:label="us-gaap_DerivativeAssetFairValueGrossLiability" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeAssetSecuritiesPurchasedUnderAgreementsToResellSecuritiesBorrowedCollateralObligationToReturnCash" xlink:label="us-gaap_DerivativeAssetSecuritiesPurchasedUnderAgreementsToResellSecuritiesBorrowedCollateralObligationToReturnCash" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeAssetSecuritiesPurchasedUnderAgreementsToResellSecuritiesBorrowedCollateralObligationToReturnSecurities" xlink:label="us-gaap_DerivativeAssetSecuritiesPurchasedUnderAgreementsToResellSecuritiesBorrowedCollateralObligationToReturnSecurities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeAssetSecuritiesPurchasedUnderAgreementsToResellSecuritiesBorrowedGross" xlink:label="us-gaap_DerivativeAssetSecuritiesPurchasedUnderAgreementsToResellSecuritiesBorrowedGross" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeAssetSecuritiesPurchasedUnderAgreementsToResellSecuritiesBorrowedLiability" xlink:label="us-gaap_DerivativeAssetSecuritiesPurchasedUnderAgreementsToResellSecuritiesBorrowedLiability" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeAssetSecurityPurchasedUnderAgreementToResellSecurityBorrowedAfterOffsetAndDeductionSubjectToMasterNettingArrangement" xlink:label="us-gaap_DerivativeAssetSecurityPurchasedUnderAgreementToResellSecurityBorrowedAfterOffsetAndDeductionSubjectToMasterNettingArrangement" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeAssetSecurityPurchasedUnderAgreementToResellSecurityBorrowedAfterOffsetSubjectToMasterNettingArrangement" xlink:label="us-gaap_DerivativeAssetSecurityPurchasedUnderAgreementToResellSecurityBorrowedAfterOffsetSubjectToMasterNettingArrangement" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeCollateralObligationToReturnCash" xlink:label="us-gaap_DerivativeCollateralObligationToReturnCash" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeCollateralObligationToReturnSecurities" xlink:label="us-gaap_DerivativeCollateralObligationToReturnSecurities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeCollateralRightToReclaimCash" xlink:label="us-gaap_DerivativeCollateralRightToReclaimCash" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeCollateralRightToReclaimSecurities" xlink:label="us-gaap_DerivativeCollateralRightToReclaimSecurities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeFairValueOfDerivativeAsset" xlink:label="us-gaap_DerivativeFairValueOfDerivativeAsset" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral" xlink:label="us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeFairValueOfDerivativeLiability" xlink:label="us-gaap_DerivativeFairValueOfDerivativeLiability" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral" xlink:label="us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeLiabilityFairValueAfterOffsetAndDeductionSubjectToMasterNettingArrangement" xlink:label="us-gaap_DerivativeLiabilityFairValueAfterOffsetAndDeductionSubjectToMasterNettingArrangement" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeLiabilityFairValueGrossAsset" xlink:label="us-gaap_DerivativeLiabilityFairValueGrossAsset" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAsset" xlink:label="us-gaap_DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAsset" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimCash" xlink:label="us-gaap_DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimCash" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimSecurities" xlink:label="us-gaap_DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimSecurities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedGross" xlink:label="us-gaap_DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedGross" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeLiabilitySecuritySoldUnderAgreementToRepurchaseSecurityLoanedAfterOffsetAndDeductionSubjectToMasterNettingArrangement" xlink:label="us-gaap_DerivativeLiabilitySecuritySoldUnderAgreementToRepurchaseSecurityLoanedAfterOffsetAndDeductionSubjectToMasterNettingArrangement" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeLiabilitySecuritySoldUnderAgreementToRepurchaseSecurityLoanedAfterOffsetSubjectToMasterNettingArrangement" xlink:label="us-gaap_DerivativeLiabilitySecuritySoldUnderAgreementToRepurchaseSecurityLoanedAfterOffsetSubjectToMasterNettingArrangement" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeTable" xlink:label="us-gaap_DerivativeTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestRateSwapMember" xlink:label="us-gaap_InterestRateSwapMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseAsset" xlink:label="us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseAsset" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimCash" xlink:label="us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimCash" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimSecurities" xlink:label="us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimSecurities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseGross" xlink:label="us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseGross" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuritySoldUnderAgreementToRepurchaseAfterOffsetAndDeductionSubjectToMasterNettingArrangement" xlink:label="us-gaap_SecuritySoldUnderAgreementToRepurchaseAfterOffsetAndDeductionSubjectToMasterNettingArrangement" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuritySoldUnderAgreementToRepurchaseAfterOffsetSubjectToMasterNettingArrangement" xlink:label="us-gaap_SecuritySoldUnderAgreementToRepurchaseAfterOffsetSubjectToMasterNettingArrangement" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DerivativeTable" xlink:to="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="us-gaap_InterestRateSwapMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DerivativeTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeFairValueOfDerivativeAsset" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeAssetFairValueGrossLiability" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeCollateralObligationToReturnSecurities" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeCollateralObligationToReturnCash" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeAssetFairValueAfterOffsetAndDeductionSubjectToMasterNettingArrangement" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeAssetSecuritiesPurchasedUnderAgreementsToResellSecuritiesBorrowedGross" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeAssetSecuritiesPurchasedUnderAgreementsToResellSecuritiesBorrowedLiability" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeAssetSecurityPurchasedUnderAgreementToResellSecurityBorrowedAfterOffsetSubjectToMasterNettingArrangement" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeAssetSecuritiesPurchasedUnderAgreementsToResellSecuritiesBorrowedCollateralObligationToReturnSecurities" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeAssetSecuritiesPurchasedUnderAgreementsToResellSecuritiesBorrowedCollateralObligationToReturnCash" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeAssetSecurityPurchasedUnderAgreementToResellSecurityBorrowedAfterOffsetAndDeductionSubjectToMasterNettingArrangement" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeFairValueOfDerivativeLiability" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeLiabilityFairValueGrossAsset" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeCollateralRightToReclaimSecurities" xlink:type="arc"/>
    <link:definitionArc order="17" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeCollateralRightToReclaimCash" xlink:type="arc"/>
    <link:definitionArc order="18" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeLiabilityFairValueAfterOffsetAndDeductionSubjectToMasterNettingArrangement" xlink:type="arc"/>
    <link:definitionArc order="19" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseGross" xlink:type="arc"/>
    <link:definitionArc order="20" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseAsset" xlink:type="arc"/>
    <link:definitionArc order="21" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SecuritySoldUnderAgreementToRepurchaseAfterOffsetSubjectToMasterNettingArrangement" xlink:type="arc"/>
    <link:definitionArc order="22" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimSecurities" xlink:type="arc"/>
    <link:definitionArc order="23" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseCollateralRightToReclaimCash" xlink:type="arc"/>
    <link:definitionArc order="24" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SecuritySoldUnderAgreementToRepurchaseAfterOffsetAndDeductionSubjectToMasterNettingArrangement" xlink:type="arc"/>
    <link:definitionArc order="25" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedGross" xlink:type="arc"/>
    <link:definitionArc order="26" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedAsset" xlink:type="arc"/>
    <link:definitionArc order="27" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeLiabilitySecuritySoldUnderAgreementToRepurchaseSecurityLoanedAfterOffsetSubjectToMasterNettingArrangement" xlink:type="arc"/>
    <link:definitionArc order="28" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimSecurities" xlink:type="arc"/>
    <link:definitionArc order="29" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeLiabilitySecuritiesSoldUnderAgreementsToResellSecuritiesLoanedCollateralRightToReclaimCash" xlink:type="arc"/>
    <link:definitionArc order="30" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeLiabilitySecuritySoldUnderAgreementToRepurchaseSecurityLoanedAfterOffsetAndDeductionSubjectToMasterNettingArrangement" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-18-regulatory-matters-summary-of-actual-capital-amounts-and-ratios-details" xlink:href="istr-20251231.xsd#statement-note-18-regulatory-matters-summary-of-actual-capital-amounts-and-ratios-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-18-regulatory-matters-summary-of-actual-capital-amounts-and-ratios-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_BankingRegulationCommonEquityTierOneRiskbasedCapitalCapitalAdequacyMinimumIncludingConservationBuffer" xlink:label="istr_BankingRegulationCommonEquityTierOneRiskbasedCapitalCapitalAdequacyMinimumIncludingConservationBuffer" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_BankingRegulationCommonEquityTierOneRiskbasedCapitalRatioCapitalAdequacyMinimumIncludingConservationBuffer" xlink:label="istr_BankingRegulationCommonEquityTierOneRiskbasedCapitalRatioCapitalAdequacyMinimumIncludingConservationBuffer" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_BankingRegulationTierOneLeverageCapitalCapitalAdequacyMinimumIncludingConservationBuffer" xlink:label="istr_BankingRegulationTierOneLeverageCapitalCapitalAdequacyMinimumIncludingConservationBuffer" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_BankingRegulationTierOneLeverageCapitalRatioCapitalAdequacyMinimumIncludingConservationBuffer" xlink:label="istr_BankingRegulationTierOneLeverageCapitalRatioCapitalAdequacyMinimumIncludingConservationBuffer" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_BankingRegulationTierOneRiskbasedCapitalCapitalAdequacyMinimumIncludingConservationBuffer" xlink:label="istr_BankingRegulationTierOneRiskbasedCapitalCapitalAdequacyMinimumIncludingConservationBuffer" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_BankingRegulationTierOneRiskbasedCapitalRatioCapitalAdequacyMinimumIncludingConservationBuffer" xlink:label="istr_BankingRegulationTierOneRiskbasedCapitalRatioCapitalAdequacyMinimumIncludingConservationBuffer" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_BankingRegulationTotalRiskbasedCapitalCapitalAdequacyMinimumIncludingConservationBuffer" xlink:label="istr_BankingRegulationTotalRiskbasedCapitalCapitalAdequacyMinimumIncludingConservationBuffer" xlink:type="locator"/>
    <link:loc xlink:href="istr-20251231.xsd#istr_BankingRegulationTotalRiskbasedCapitalRatioCapitalAdequacyMinimumIncludingConservationBuffer" xlink:label="istr_BankingRegulationTotalRiskbasedCapitalRatioCapitalAdequacyMinimumIncludingConservationBuffer" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesAxis" xlink:label="srt_ConsolidatedEntitiesAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain" xlink:label="srt_ConsolidatedEntitiesDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain" xlink:label="srt_ConsolidatedEntitiesDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ParentCompanyMember" xlink:label="srt_ParentCompanyMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramTable" xlink:label="srt_ShareRepurchaseProgramTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SubsidiariesMember" xlink:label="srt_SubsidiariesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BankingRegulationCommonEquityTierOneRiskBasedCapitalRatioWellCapitalizedMinimum" xlink:label="us-gaap_BankingRegulationCommonEquityTierOneRiskBasedCapitalRatioWellCapitalizedMinimum" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Capital" xlink:label="us-gaap_Capital" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalRequiredToBeWellCapitalized" xlink:label="us-gaap_CapitalRequiredToBeWellCapitalized" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets" xlink:label="us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalToRiskWeightedAssets" xlink:label="us-gaap_CapitalToRiskWeightedAssets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonEquityTierOneCapital" xlink:label="us-gaap_CommonEquityTierOneCapital" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonEquityTierOneCapitalRatio" xlink:label="us-gaap_CommonEquityTierOneCapitalRatio" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonEquityTierOneCapitalRequiredToBeWellCapitalized" xlink:label="us-gaap_CommonEquityTierOneCapitalRequiredToBeWellCapitalized" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneLeverageCapital" xlink:label="us-gaap_TierOneLeverageCapital" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized" xlink:label="us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets" xlink:label="us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneLeverageCapitalToAverageAssets" xlink:label="us-gaap_TierOneLeverageCapitalToAverageAssets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneRiskBasedCapital" xlink:label="us-gaap_TierOneRiskBasedCapital" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized" xlink:label="us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets" xlink:label="us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets" xlink:label="us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_ConsolidatedEntitiesAxis" xlink:to="srt_ConsolidatedEntitiesDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="srt_ShareRepurchaseProgramTable" xlink:to="srt_ConsolidatedEntitiesAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_ConsolidatedEntitiesDomain" xlink:to="srt_ParentCompanyMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_ConsolidatedEntitiesAxis" xlink:to="srt_ConsolidatedEntitiesDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_ConsolidatedEntitiesDomain" xlink:to="srt_SubsidiariesMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="srt_ShareRepurchaseProgramTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="srt_ShareRepurchaseProgramTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TierOneLeverageCapital" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TierOneLeverageCapitalToAverageAssets" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_BankingRegulationTierOneLeverageCapitalCapitalAdequacyMinimumIncludingConservationBuffer" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_BankingRegulationTierOneLeverageCapitalRatioCapitalAdequacyMinimumIncludingConservationBuffer" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonEquityTierOneCapital" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonEquityTierOneCapitalRatio" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_BankingRegulationCommonEquityTierOneRiskbasedCapitalCapitalAdequacyMinimumIncludingConservationBuffer" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_BankingRegulationCommonEquityTierOneRiskbasedCapitalRatioCapitalAdequacyMinimumIncludingConservationBuffer" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonEquityTierOneCapitalRequiredToBeWellCapitalized" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_BankingRegulationCommonEquityTierOneRiskBasedCapitalRatioWellCapitalizedMinimum" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TierOneRiskBasedCapital" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_BankingRegulationTierOneRiskbasedCapitalCapitalAdequacyMinimumIncludingConservationBuffer" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_BankingRegulationTierOneRiskbasedCapitalRatioCapitalAdequacyMinimumIncludingConservationBuffer" xlink:type="arc"/>
    <link:definitionArc order="17" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized" xlink:type="arc"/>
    <link:definitionArc order="18" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets" xlink:type="arc"/>
    <link:definitionArc order="19" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_Capital" xlink:type="arc"/>
    <link:definitionArc order="20" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CapitalToRiskWeightedAssets" xlink:type="arc"/>
    <link:definitionArc order="21" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_BankingRegulationTotalRiskbasedCapitalCapitalAdequacyMinimumIncludingConservationBuffer" xlink:type="arc"/>
    <link:definitionArc order="22" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="istr_BankingRegulationTotalRiskbasedCapitalRatioCapitalAdequacyMinimumIncludingConservationBuffer" xlink:type="arc"/>
    <link:definitionArc order="23" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CapitalRequiredToBeWellCapitalized" xlink:type="arc"/>
    <link:definitionArc order="24" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments-assets-and-liabilities-measured-at-fair-value-on-recurring-basis-details" xlink:href="istr-20251231.xsd#statement-note-17-fair-values-of-financial-instruments-assets-and-liabilities-measured-at-fair-value-on-recurring-basis-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://investors.investarbank.com/20251231/role/statement-note-17-fair-values-of-financial-instruments-assets-and-liabilities-measured-at-fair-value-on-recurring-basis-details" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsFairValueDisclosure" xlink:label="us-gaap_AssetsFairValueDisclosure" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialMortgageBackedSecuritiesMember" xlink:label="us-gaap_CommercialMortgageBackedSecuritiesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateDebtSecuritiesMember" xlink:label="us-gaap_CorporateDebtSecuritiesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeAssets" xlink:label="us-gaap_DerivativeAssets" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeLiabilities" xlink:label="us-gaap_DerivativeLiabilities" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityFairValueDisclosure" xlink:label="us-gaap_EquityFairValueDisclosure" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="us-gaap_FairValueInputsLevel1Member" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="us-gaap_FairValueInputsLevel2Member" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="us-gaap_FairValueInputsLevel3Member" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:label="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestRateSwapMember" xlink:label="us-gaap_InterestRateSwapMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResidentialMortgageBackedSecuritiesMember" xlink:label="us-gaap_ResidentialMortgageBackedSecuritiesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember" xlink:label="us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:label="us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:to="us-gaap_FinancialInstrumentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:to="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel1Member" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel2Member" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel3Member" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_CorporateDebtSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_ResidentialMortgageBackedSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_CommercialMortgageBackedSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:to="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="us-gaap_InterestRateSwapMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EquityFairValueDisclosure" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeAssets" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AssetsFairValueDisclosure" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeLiabilities" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-roles-2025.xsd#eedm" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="extended">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract" xlink:label="us-gaap_AssetsAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestReceivable" xlink:label="us-gaap_InterestReceivable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="false" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_InterestReceivable" xlink:type="arc"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssets" xlink:label="us-gaap_OtherAssets" xlink:type="locator"/>
    <link:definitionArc order="1" xbrldt:closed="false" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_OtherAssets" xlink:type="arc"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAbstract" xlink:label="us-gaap_LiabilitiesAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent" xlink:label="us-gaap_AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent" xlink:type="locator"/>
    <link:definitionArc order="2" xbrldt:closed="false" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAbstract" xlink:to="us-gaap_AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent" xlink:type="arc"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementAbstract" xlink:label="us-gaap_IncomeStatementAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoninterestIncomeOtherOperatingIncome" xlink:label="us-gaap_NoninterestIncomeOtherOperatingIncome" xlink:type="locator"/>
    <link:definitionArc order="3" xbrldt:closed="false" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeStatementAbstract" xlink:to="us-gaap_NoninterestIncomeOtherOperatingIncome" xlink:type="arc"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementOfOtherComprehensiveIncomeAbstract" xlink:label="us-gaap_StatementOfOtherComprehensiveIncomeAbstract" xlink:type="locator"/>
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" xlink:label="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" xlink:type="locator"/>
    <link:definitionArc order="4" xbrldt:closed="false" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementOfOtherComprehensiveIncomeAbstract" xlink:to="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" xlink:type="arc"/>
  </link:definitionLink>
</link:linkbase>
