<?xml version="1.0" encoding="utf-8"?>
<!--XBRL document created with Toppan Merrill Bridge  11.3.0.1 -->
<!--Based on XBRL 2.1-->
<!--Created on: 5/5/2026 10:56:52 AM-->
<!--Modified on: 5/5/2026 10:56:52 AM-->
<link:linkbase xmlns="http://www.xbrl.org/2003/linkbase" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xbrli="http://www.xbrl.org/2003/instance">
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DocumentDocumentAndEntityInformation" roleURI="http://selectenergyservices.com/role/DocumentDocumentAndEntityInformation" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#StatementConsolidatedBalanceSheets" roleURI="http://selectenergyservices.com/role/StatementConsolidatedBalanceSheets" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#StatementConsolidatedBalanceSheetsParenthetical" roleURI="http://selectenergyservices.com/role/StatementConsolidatedBalanceSheetsParenthetical" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#StatementConsolidatedStatementsOfOperations" roleURI="http://selectenergyservices.com/role/StatementConsolidatedStatementsOfOperations" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#StatementConsolidatedStatementsOfChangesInEquity" roleURI="http://selectenergyservices.com/role/StatementConsolidatedStatementsOfChangesInEquity" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#StatementConsolidatedStatementsOfChangesInEquityParenthetical" roleURI="http://selectenergyservices.com/role/StatementConsolidatedStatementsOfChangesInEquityParenthetical" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureAcquisitionsTables" roleURI="http://selectenergyservices.com/role/DisclosureAcquisitionsTables" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureEquityBasedCompensationTables" roleURI="http://selectenergyservices.com/role/DisclosureEquityBasedCompensationTables" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureBusinessAndBasisOfPresentationDetails" roleURI="http://selectenergyservices.com/role/DisclosureBusinessAndBasisOfPresentationDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureSignificantAccountingPoliciesEquityInvestmentsDetails" roleURI="http://selectenergyservices.com/role/DisclosureSignificantAccountingPoliciesEquityInvestmentsDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureSignificantAccountingPoliciesInvestmentsInUnconsolidatedDetails" roleURI="http://selectenergyservices.com/role/DisclosureSignificantAccountingPoliciesInvestmentsInUnconsolidatedDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureAcquisitionsDetails" roleURI="http://selectenergyservices.com/role/DisclosureAcquisitionsDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureAcquisitionsBusinessCombinationDetails" roleURI="http://selectenergyservices.com/role/DisclosureAcquisitionsBusinessCombinationDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureAcquisitionsConsiderationTransferredAndEstimatedFairValueOfIdentifiedAssetsLiabilitiesDetails" roleURI="http://selectenergyservices.com/role/DisclosureAcquisitionsConsiderationTransferredAndEstimatedFairValueOfIdentifiedAssetsLiabilitiesDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureAcquisitionsAssetAcquisitionsDetails" roleURI="http://selectenergyservices.com/role/DisclosureAcquisitionsAssetAcquisitionsDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureRevenueDetails" roleURI="http://selectenergyservices.com/role/DisclosureRevenueDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureInventoriesDetails" roleURI="http://selectenergyservices.com/role/DisclosureInventoriesDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosurePropertyAndEquipmentDetails" roleURI="http://selectenergyservices.com/role/DisclosurePropertyAndEquipmentDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureGoodwillAndOtherIntangibleAssetsChangesInCarryingAmountsOfGoodwillDetails" roleURI="http://selectenergyservices.com/role/DisclosureGoodwillAndOtherIntangibleAssetsChangesInCarryingAmountsOfGoodwillDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureGoodwillAndOtherIntangibleAssetsOtherIntangibleAssetsDetails" roleURI="http://selectenergyservices.com/role/DisclosureGoodwillAndOtherIntangibleAssetsOtherIntangibleAssetsDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureGoodwillAndOtherIntangibleAssetsOtherIntangibleAssetsNarrativeDetails" roleURI="http://selectenergyservices.com/role/DisclosureGoodwillAndOtherIntangibleAssetsOtherIntangibleAssetsNarrativeDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureDebtDetails" roleURI="http://selectenergyservices.com/role/DisclosureDebtDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureDebtPrincipalMaturitiesOfDebtOutstandingDetails" roleURI="http://selectenergyservices.com/role/DisclosureDebtPrincipalMaturitiesOfDebtOutstandingDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureEquityBasedCompensationDetails" roleURI="http://selectenergyservices.com/role/DisclosureEquityBasedCompensationDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureEquityBasedCompensationEquityOptionsChangedDuringPeriodDetails" roleURI="http://selectenergyservices.com/role/DisclosureEquityBasedCompensationEquityOptionsChangedDuringPeriodDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureEquityBasedCompensationEquityOptionsDetails" roleURI="http://selectenergyservices.com/role/DisclosureEquityBasedCompensationEquityOptionsDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureEquityBasedCompensationRestrictedStockAwardsDetails" roleURI="http://selectenergyservices.com/role/DisclosureEquityBasedCompensationRestrictedStockAwardsDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureEquityBasedCompensationPerformanceShareUnitsDetails" roleURI="http://selectenergyservices.com/role/DisclosureEquityBasedCompensationPerformanceShareUnitsDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureEquityBasedCompensationPerformanceShareUnitsOutstandingDetails" roleURI="http://selectenergyservices.com/role/DisclosureEquityBasedCompensationPerformanceShareUnitsOutstandingDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureEquityBasedCompensationShareRepurchasesDetails" roleURI="http://selectenergyservices.com/role/DisclosureEquityBasedCompensationShareRepurchasesDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureRelatedPartyTransactionsDetails" roleURI="http://selectenergyservices.com/role/DisclosureRelatedPartyTransactionsDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureNoncontrollingInterestsEffectOfChangesDetails" roleURI="http://selectenergyservices.com/role/DisclosureNoncontrollingInterestsEffectOfChangesDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureIncomePerShareDetails" roleURI="http://selectenergyservices.com/role/DisclosureIncomePerShareDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureSegmentInformationDetails" roleURI="http://selectenergyservices.com/role/DisclosureSegmentInformationDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureSegmentInformationTotalAssetsDetails" roleURI="http://selectenergyservices.com/role/DisclosureSegmentInformationTotalAssetsDetails" />
  <link:roleRef xlink:type="simple" xlink:href="wttr-20260331.xsd#DisclosureSubsequentEventsDetails" roleURI="http://selectenergyservices.com/role/DisclosureSubsequentEventsDetails" />
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:href="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd#IndividualsOnly" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://www.esma.europa.eu/xbrl/esef/arcrole/wider-narrower" xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/arcrole/esma-arcrole-2018-11-21.xsd#wider-narrower" />
  <link:roleRef roleURI="http://selectenergyservices.com/role/DisclosureStandardAndCustomAxisDomainDefaults" xlink:href="wttr-20260331.xsd#DisclosureStandardAndCustomAxisDomainDefaults" xlink:type="simple" />
  <link:roleRef xlink:type="simple" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-roles-2025.xsd#eedm" roleURI="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" />
  <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" />
  <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" />
  <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" />
  <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" />
  <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" arcroleURI="http://xbrl.org/int/dim/arcrole/all" />
  <link:definitionLink xlink:role="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="extended" xlink:title="Extensible Enumeration Domain Members">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssetsNoncurrent" xlink:label="us-gaap_OtherAssetsNoncurrent_1" xlink:title="us-gaap_OtherAssetsNoncurrent_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsNoncurrentAbstract" xlink:label="us-gaap_AssetsNoncurrentAbstract" xlink:title="us-gaap_AssetsNoncurrentAbstract" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill" xlink:label="us-gaap_Goodwill_1" xlink:title="us-gaap_Goodwill_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="us-gaap_CashAndCashEquivalentsAtCarryingValue_1" xlink:title="us-gaap_CashAndCashEquivalentsAtCarryingValue_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract" xlink:label="us-gaap_AssetsAbstract_1" xlink:title="us-gaap_AssetsAbstract_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="us-gaap_PropertyPlantAndEquipmentNet_1" xlink:title="us-gaap_PropertyPlantAndEquipmentNet_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_AssetsAbstract_1" xlink:to="us-gaap_PropertyPlantAndEquipmentNet_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_AssetsAbstract_1 To us-gaap_PropertyPlantAndEquipmentNet_1" order="2" priority="0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAsset" xlink:label="us-gaap_OperatingLeaseRightOfUseAsset_1" xlink:title="us-gaap_OperatingLeaseRightOfUseAsset_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_AssetsAbstract_1" xlink:to="us-gaap_OperatingLeaseRightOfUseAsset_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_AssetsAbstract_1 To us-gaap_OperatingLeaseRightOfUseAsset_1" order="1" priority="0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrentAbstract" xlink:label="us-gaap_LiabilitiesCurrentAbstract_1" xlink:title="us-gaap_LiabilitiesCurrentAbstract_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityCurrent" xlink:label="us-gaap_OperatingLeaseLiabilityCurrent_1" xlink:title="us-gaap_OperatingLeaseLiabilityCurrent_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_LiabilitiesCurrentAbstract_1" xlink:to="us-gaap_OperatingLeaseLiabilityCurrent_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_LiabilitiesCurrentAbstract_1 To us-gaap_OperatingLeaseLiabilityCurrent_1" order="1" priority="0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinanceLeaseLiabilityCurrent" xlink:label="us-gaap_FinanceLeaseLiabilityCurrent_1" xlink:title="us-gaap_FinanceLeaseLiabilityCurrent_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_LiabilitiesCurrentAbstract_1" xlink:to="us-gaap_FinanceLeaseLiabilityCurrent_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_LiabilitiesCurrentAbstract_1 To us-gaap_FinanceLeaseLiabilityCurrent_1" order="2" priority="0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesNoncurrentAbstract" xlink:label="us-gaap_LiabilitiesNoncurrentAbstract" xlink:title="us-gaap_LiabilitiesNoncurrentAbstract" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent" xlink:label="us-gaap_OperatingLeaseLiabilityNoncurrent_1" xlink:title="us-gaap_OperatingLeaseLiabilityNoncurrent_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_LiabilitiesNoncurrentAbstract" xlink:to="us-gaap_OperatingLeaseLiabilityNoncurrent_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_LiabilitiesNoncurrentAbstract To us-gaap_OperatingLeaseLiabilityNoncurrent_1" order="1" priority="0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAbstract" xlink:label="us-gaap_LiabilitiesAbstract" xlink:title="us-gaap_LiabilitiesAbstract" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiability" xlink:label="us-gaap_OperatingLeaseLiability" xlink:title="us-gaap_OperatingLeaseLiability" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_LiabilitiesAbstract" xlink:to="us-gaap_OperatingLeaseLiability" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_LiabilitiesAbstract To us-gaap_OperatingLeaseLiability" order="1" priority="0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementAbstract" xlink:label="us-gaap_IncomeStatementAbstract" xlink:title="us-gaap_IncomeStatementAbstract" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillAndIntangibleAssetImpairment" xlink:label="us-gaap_GoodwillAndIntangibleAssetImpairment_2" xlink:title="us-gaap_GoodwillAndIntangibleAssetImpairment_2" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_IncomeStatementAbstract" xlink:to="us-gaap_GoodwillAndIntangibleAssetImpairment_2" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_IncomeStatementAbstract To us-gaap_GoodwillAndIntangibleAssetImpairment_2" order="1" priority="0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure" xlink:label="us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure_1" xlink:title="us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_IncomeStatementAbstract" xlink:to="us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_IncomeStatementAbstract To us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure_1" order="4" priority="0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentsFairValueDisclosure" xlink:label="us-gaap_InvestmentsFairValueDisclosure_1" xlink:title="us-gaap_InvestmentsFairValueDisclosure_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_IncomeStatementAbstract" xlink:to="us-gaap_InvestmentsFairValueDisclosure_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_IncomeStatementAbstract To us-gaap_InvestmentsFairValueDisclosure_1" order="3" priority="0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentFairValueDisclosure" xlink:label="us-gaap_PropertyPlantAndEquipmentFairValueDisclosure_1" xlink:title="us-gaap_PropertyPlantAndEquipmentFairValueDisclosure_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_IncomeStatementAbstract" xlink:to="us-gaap_PropertyPlantAndEquipmentFairValueDisclosure_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_IncomeStatementAbstract To us-gaap_PropertyPlantAndEquipmentFairValueDisclosure_1" order="2" priority="0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent" xlink:label="us-gaap_OperatingLeaseLiabilityNoncurrent_3" xlink:title="us-gaap_OperatingLeaseLiabilityNoncurrent_3" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_LiabilitiesAbstract" xlink:to="us-gaap_OperatingLeaseLiabilityNoncurrent_3" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_LiabilitiesAbstract To us-gaap_OperatingLeaseLiabilityNoncurrent_3" order="2" priority="0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityCurrent" xlink:label="us-gaap_OperatingLeaseLiabilityCurrent_2" xlink:title="us-gaap_OperatingLeaseLiabilityCurrent_2" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_LiabilitiesAbstract" xlink:to="us-gaap_OperatingLeaseLiabilityCurrent_2" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_LiabilitiesAbstract To us-gaap_OperatingLeaseLiabilityCurrent_2" order="3" priority="0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="us-gaap_IncomeTaxAuthorityDomain_3" xlink:title="us-gaap_IncomeTaxAuthorityDomain_3" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd#stpr_CO" xlink:label="stpr_CO" xlink:title="stpr_CO" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_IncomeTaxAuthorityDomain_3" xlink:to="stpr_CO" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_IncomeTaxAuthorityDomain_3 To stpr_CO" order="15" priority="0" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureStandardAndCustomAxisDomainDefaults" xlink:type="extended" xlink:title="99900 - Disclosure - Standard And Custom Axis Domain Defaults">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis_1" xlink:title="us-gaap_StatementClassOfStockAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain_1" xlink:title="us-gaap_ClassOfStockDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_StatementClassOfStockAxis_1" xlink:to="us-gaap_ClassOfStockDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_StatementClassOfStockAxis_1 To us-gaap_ClassOfStockDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementScenarioAxis" xlink:label="srt_StatementScenarioAxis_1" xlink:title="srt_StatementScenarioAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="srt_ScenarioUnspecifiedDomain_1" xlink:title="srt_ScenarioUnspecifiedDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_StatementScenarioAxis_1" xlink:to="srt_ScenarioUnspecifiedDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_StatementScenarioAxis_1 To srt_ScenarioUnspecifiedDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis" xlink:label="srt_ProductOrServiceAxis_1" xlink:title="srt_ProductOrServiceAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain" xlink:label="srt_ProductsAndServicesDomain_1" xlink:title="srt_ProductsAndServicesDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_ProductOrServiceAxis_1" xlink:to="srt_ProductsAndServicesDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_ProductOrServiceAxis_1 To srt_ProductsAndServicesDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConversionOfStockByUniqueDescriptionAxis" xlink:label="us-gaap_ConversionOfStockByUniqueDescriptionAxis_1" xlink:title="us-gaap_ConversionOfStockByUniqueDescriptionAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConversionOfStockNameDomain" xlink:label="us-gaap_ConversionOfStockNameDomain_1" xlink:title="us-gaap_ConversionOfStockNameDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_ConversionOfStockByUniqueDescriptionAxis_1" xlink:to="us-gaap_ConversionOfStockNameDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_ConversionOfStockByUniqueDescriptionAxis_1 To us-gaap_ConversionOfStockNameDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="us-gaap_StatementEquityComponentsAxis_1" xlink:title="us-gaap_StatementEquityComponentsAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain_1" xlink:title="us-gaap_EquityComponentDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_StatementEquityComponentsAxis_1" xlink:to="us-gaap_EquityComponentDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_StatementEquityComponentsAxis_1 To us-gaap_EquityComponentDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis_1" xlink:title="dei_LegalEntityAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain_1" xlink:title="dei_EntityDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="dei_LegalEntityAxis_1" xlink:to="dei_EntityDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:dei_LegalEntityAxis_1 To dei_EntityDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="us-gaap_BusinessAcquisitionAxis_1" xlink:title="us-gaap_BusinessAcquisitionAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain_1" xlink:title="us-gaap_BusinessAcquisitionAcquireeDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_BusinessAcquisitionAxis_1" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_BusinessAcquisitionAxis_1 To us-gaap_BusinessAcquisitionAcquireeDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis_1" xlink:title="us-gaap_AwardTypeAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1" xlink:title="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_AwardTypeAxis_1" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_AwardTypeAxis_1 To us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesAxis" xlink:label="srt_ConsolidatedEntitiesAxis_1" xlink:title="srt_ConsolidatedEntitiesAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain" xlink:label="srt_ConsolidatedEntitiesDomain_1" xlink:title="srt_ConsolidatedEntitiesDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_ConsolidatedEntitiesAxis_1" xlink:to="srt_ConsolidatedEntitiesDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_ConsolidatedEntitiesAxis_1 To srt_ConsolidatedEntitiesDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="us-gaap_SubsidiarySaleOfStockAxis_1" xlink:title="us-gaap_SubsidiarySaleOfStockAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain_1" xlink:title="us-gaap_SaleOfStockNameOfTransactionDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_SubsidiarySaleOfStockAxis_1" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_SubsidiarySaleOfStockAxis_1 To us-gaap_SaleOfStockNameOfTransactionDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:label="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_1" xlink:title="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="srt_EquityMethodInvesteeNameDomain_1" xlink:title="srt_EquityMethodInvesteeNameDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_1" xlink:to="srt_EquityMethodInvesteeNameDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_1 To srt_EquityMethodInvesteeNameDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="us-gaap_BalanceSheetLocationAxis_1" xlink:title="us-gaap_BalanceSheetLocationAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="us-gaap_BalanceSheetLocationDomain_1" xlink:title="us-gaap_BalanceSheetLocationDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_BalanceSheetLocationAxis_1" xlink:to="us-gaap_BalanceSheetLocationDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_BalanceSheetLocationAxis_1 To us-gaap_BalanceSheetLocationDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionAxis" xlink:label="us-gaap_RelatedPartyTransactionAxis_1" xlink:title="us-gaap_RelatedPartyTransactionAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="us-gaap_RelatedPartyTransactionDomain_1" xlink:title="us-gaap_RelatedPartyTransactionDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_RelatedPartyTransactionAxis_1" xlink:to="us-gaap_RelatedPartyTransactionDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_RelatedPartyTransactionAxis_1 To us-gaap_RelatedPartyTransactionDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByTypeAxis" xlink:label="us-gaap_ConcentrationRiskByTypeAxis_1" xlink:title="us-gaap_ConcentrationRiskByTypeAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="us-gaap_ConcentrationRiskTypeDomain_1" xlink:title="us-gaap_ConcentrationRiskTypeDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_ConcentrationRiskByTypeAxis_1" xlink:to="us-gaap_ConcentrationRiskTypeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_ConcentrationRiskByTypeAxis_1 To us-gaap_ConcentrationRiskTypeDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ChangeInAccountingEstimateByTypeAxis" xlink:label="us-gaap_ChangeInAccountingEstimateByTypeAxis_1" xlink:title="us-gaap_ChangeInAccountingEstimateByTypeAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ChangeInAccountingEstimateTypeDomain" xlink:label="us-gaap_ChangeInAccountingEstimateTypeDomain_1" xlink:title="us-gaap_ChangeInAccountingEstimateTypeDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_ChangeInAccountingEstimateByTypeAxis_1" xlink:to="us-gaap_ChangeInAccountingEstimateTypeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_ChangeInAccountingEstimateByTypeAxis_1 To us-gaap_ChangeInAccountingEstimateTypeDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis_1" xlink:title="us-gaap_PropertyPlantAndEquipmentByTypeAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain_1" xlink:title="us-gaap_PropertyPlantAndEquipmentTypeDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis_1" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_PropertyPlantAndEquipmentByTypeAxis_1 To us-gaap_PropertyPlantAndEquipmentTypeDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis_1" xlink:title="srt_RangeAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember_1" xlink:title="srt_RangeMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_RangeAxis_1" xlink:to="srt_RangeMember_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_RangeAxis_1 To srt_RangeMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="us-gaap_StatementBusinessSegmentsAxis_1" xlink:title="us-gaap_StatementBusinessSegmentsAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain" xlink:label="us-gaap_SegmentDomain_1" xlink:title="us-gaap_SegmentDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_StatementBusinessSegmentsAxis_1" xlink:to="us-gaap_SegmentDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_StatementBusinessSegmentsAxis_1 To us-gaap_SegmentDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ImpairedAssetsToBeDisposedOfByMethodOtherThanSaleByTypeAxis" xlink:label="us-gaap_ImpairedAssetsToBeDisposedOfByMethodOtherThanSaleByTypeAxis_1" xlink:title="us-gaap_ImpairedAssetsToBeDisposedOfByMethodOtherThanSaleByTypeAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ImpairedAssetsToBeDisposedOfByMethodOtherThanSaleAssetNameDomain" xlink:label="us-gaap_ImpairedAssetsToBeDisposedOfByMethodOtherThanSaleAssetNameDomain_1" xlink:title="us-gaap_ImpairedAssetsToBeDisposedOfByMethodOtherThanSaleAssetNameDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_ImpairedAssetsToBeDisposedOfByMethodOtherThanSaleByTypeAxis_1" xlink:to="us-gaap_ImpairedAssetsToBeDisposedOfByMethodOtherThanSaleAssetNameDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_ImpairedAssetsToBeDisposedOfByMethodOtherThanSaleByTypeAxis_1 To us-gaap_ImpairedAssetsToBeDisposedOfByMethodOtherThanSaleAssetNameDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="us-gaap_IncomeStatementLocationAxis_1" xlink:title="us-gaap_IncomeStatementLocationAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="us-gaap_IncomeStatementLocationDomain_1" xlink:title="us-gaap_IncomeStatementLocationDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_IncomeStatementLocationAxis_1" xlink:to="us-gaap_IncomeStatementLocationDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_IncomeStatementLocationAxis_1 To us-gaap_IncomeStatementLocationDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_1" xlink:title="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_1" xlink:title="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_1" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_1 To us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByAssetClassAxis" xlink:label="us-gaap_FairValueByAssetClassAxis_1" xlink:title="us-gaap_FairValueByAssetClassAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:label="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_1" xlink:title="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_FairValueByAssetClassAxis_1" xlink:to="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_FairValueByAssetClassAxis_1 To us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis" xlink:label="srt_StatementGeographicalAxis_1" xlink:title="srt_StatementGeographicalAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain" xlink:label="srt_SegmentGeographicalDomain_1" xlink:title="srt_SegmentGeographicalDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_StatementGeographicalAxis_1" xlink:to="srt_SegmentGeographicalDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_StatementGeographicalAxis_1 To srt_SegmentGeographicalDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:label="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_1" xlink:title="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="us-gaap_TypeOfAdoptionMember_1" xlink:title="us-gaap_TypeOfAdoptionMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_1" xlink:to="us-gaap_TypeOfAdoptionMember_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_1 To us-gaap_TypeOfAdoptionMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="us-gaap_LongtermDebtTypeAxis_1" xlink:title="us-gaap_LongtermDebtTypeAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="us-gaap_LongtermDebtTypeDomain_1" xlink:title="us-gaap_LongtermDebtTypeDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_LongtermDebtTypeAxis_1" xlink:to="us-gaap_LongtermDebtTypeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_LongtermDebtTypeAxis_1 To us-gaap_LongtermDebtTypeDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_1" xlink:title="us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_1" xlink:title="us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_1" xlink:to="us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_1 To us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis" xlink:label="us-gaap_CreditFacilityAxis_1" xlink:title="us-gaap_CreditFacilityAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain" xlink:label="us-gaap_CreditFacilityDomain_1" xlink:title="us-gaap_CreditFacilityDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_CreditFacilityAxis_1" xlink:to="us-gaap_CreditFacilityDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_CreditFacilityAxis_1 To us-gaap_CreditFacilityDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeAxis" xlink:label="us-gaap_ShortTermDebtTypeAxis_1" xlink:title="us-gaap_ShortTermDebtTypeAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeDomain" xlink:label="us-gaap_ShortTermDebtTypeDomain_1" xlink:title="us-gaap_ShortTermDebtTypeDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_ShortTermDebtTypeAxis_1" xlink:to="us-gaap_ShortTermDebtTypeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_ShortTermDebtTypeAxis_1 To us-gaap_ShortTermDebtTypeDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BorrowingBaseAxis" xlink:label="wttr_BorrowingBaseAxis_1" xlink:title="wttr_BorrowingBaseAxis_1" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BorrowingBaseDomain" xlink:label="wttr_BorrowingBaseDomain_1" xlink:title="wttr_BorrowingBaseDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="wttr_BorrowingBaseAxis_1" xlink:to="wttr_BorrowingBaseDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:wttr_BorrowingBaseAxis_1 To wttr_BorrowingBaseDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_FinancialCovenantsAndRestrictionAxis" xlink:label="wttr_FinancialCovenantsAndRestrictionAxis_1" xlink:title="wttr_FinancialCovenantsAndRestrictionAxis_1" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_FinancialCovenantsAndRestrictionDomain" xlink:label="wttr_FinancialCovenantsAndRestrictionDomain_1" xlink:title="wttr_FinancialCovenantsAndRestrictionDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="wttr_FinancialCovenantsAndRestrictionAxis_1" xlink:to="wttr_FinancialCovenantsAndRestrictionDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:wttr_FinancialCovenantsAndRestrictionAxis_1 To wttr_FinancialCovenantsAndRestrictionDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateAxis" xlink:label="us-gaap_VariableRateAxis_1" xlink:title="us-gaap_VariableRateAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain" xlink:label="us-gaap_VariableRateDomain_1" xlink:title="us-gaap_VariableRateDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_VariableRateAxis_1" xlink:to="us-gaap_VariableRateDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_VariableRateAxis_1 To us-gaap_VariableRateDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="us-gaap_SubsequentEventTypeAxis_1" xlink:title="us-gaap_SubsequentEventTypeAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="us-gaap_SubsequentEventTypeDomain_1" xlink:title="us-gaap_SubsequentEventTypeDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_SubsequentEventTypeAxis_1" xlink:to="us-gaap_SubsequentEventTypeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_SubsequentEventTypeAxis_1 To us-gaap_SubsequentEventTypeDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestructuringCostAndReserveAxis" xlink:label="us-gaap_RestructuringCostAndReserveAxis_1" xlink:title="us-gaap_RestructuringCostAndReserveAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="us-gaap_TypeOfRestructuringDomain_1" xlink:title="us-gaap_TypeOfRestructuringDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_RestructuringCostAndReserveAxis_1" xlink:to="us-gaap_TypeOfRestructuringDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_RestructuringCostAndReserveAxis_1 To us-gaap_TypeOfRestructuringDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameAxis" xlink:label="us-gaap_PlanNameAxis_1" xlink:title="us-gaap_PlanNameAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain" xlink:label="us-gaap_PlanNameDomain_1" xlink:title="us-gaap_PlanNameDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_PlanNameAxis_1" xlink:to="us-gaap_PlanNameDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_PlanNameAxis_1 To us-gaap_PlanNameDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardDateAxis" xlink:label="us-gaap_AwardDateAxis_1" xlink:title="us-gaap_AwardDateAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardDateDomain" xlink:label="us-gaap_AwardDateDomain_1" xlink:title="us-gaap_AwardDateDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_AwardDateAxis_1" xlink:to="us-gaap_AwardDateDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_AwardDateAxis_1 To us-gaap_AwardDateDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis_1" xlink:title="us-gaap_FairValueByFairValueHierarchyLevelAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1" xlink:title="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis_1" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_FairValueByFairValueHierarchyLevelAxis_1 To us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:label="us-gaap_FairValueByMeasurementFrequencyAxis_1" xlink:title="us-gaap_FairValueByMeasurementFrequencyAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="us-gaap_FairValueMeasurementFrequencyDomain_1" xlink:title="us-gaap_FairValueMeasurementFrequencyDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_FairValueByMeasurementFrequencyAxis_1" xlink:to="us-gaap_FairValueMeasurementFrequencyDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_FairValueByMeasurementFrequencyAxis_1 To us-gaap_FairValueMeasurementFrequencyDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis_1" xlink:title="us-gaap_FinancialInstrumentAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1" xlink:title="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_FinancialInstrumentAxis_1" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_FinancialInstrumentAxis_1 To us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementBasisAxis" xlink:label="us-gaap_FairValueByMeasurementBasisAxis_1" xlink:title="us-gaap_FairValueByMeasurementBasisAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:label="us-gaap_PortionAtFairValueFairValueDisclosureMember_1" xlink:title="us-gaap_PortionAtFairValueFairValueDisclosureMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_FairValueByMeasurementBasisAxis_1" xlink:to="us-gaap_PortionAtFairValueFairValueDisclosureMember_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_FairValueByMeasurementBasisAxis_1 To us-gaap_PortionAtFairValueFairValueDisclosureMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="us-gaap_TypeOfArrangementAxis_1" xlink:title="us-gaap_TypeOfArrangementAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember_1" xlink:title="us-gaap_ArrangementsAndNonarrangementTransactionsMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_TypeOfArrangementAxis_1" xlink:to="us-gaap_ArrangementsAndNonarrangementTransactionsMember_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_TypeOfArrangementAxis_1 To us-gaap_ArrangementsAndNonarrangementTransactionsMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_1" xlink:title="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="us-gaap_RelatedPartyDomain_1" xlink:title="us-gaap_RelatedPartyDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_1" xlink:to="us-gaap_RelatedPartyDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_1 To us-gaap_RelatedPartyDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipAxis" xlink:label="srt_OwnershipAxis_1" xlink:title="srt_OwnershipAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipDomain" xlink:label="srt_OwnershipDomain_1" xlink:title="srt_OwnershipDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_OwnershipAxis_1" xlink:to="srt_OwnershipDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_OwnershipAxis_1 To srt_OwnershipDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidationItemsAxis" xlink:label="srt_ConsolidationItemsAxis_1" xlink:title="srt_ConsolidationItemsAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidationItemsDomain" xlink:label="srt_ConsolidationItemsDomain_1" xlink:title="srt_ConsolidationItemsDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_ConsolidationItemsAxis_1" xlink:to="srt_ConsolidationItemsDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_ConsolidationItemsAxis_1 To srt_ConsolidationItemsDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis" xlink:label="srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_1" xlink:title="srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain" xlink:label="srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_1" xlink:title="srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_1" xlink:to="srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_1 To srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationAxis" xlink:label="us-gaap_DisposalGroupClassificationAxis_1" xlink:title="us-gaap_DisposalGroupClassificationAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain" xlink:label="us-gaap_DisposalGroupClassificationDomain_1" xlink:title="us-gaap_DisposalGroupClassificationDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DisposalGroupClassificationAxis_1" xlink:to="us-gaap_DisposalGroupClassificationDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_DisposalGroupClassificationAxis_1 To us-gaap_DisposalGroupClassificationDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis" xlink:label="us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_1" xlink:title="us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain" xlink:label="us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_1" xlink:title="us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_1" xlink:to="us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_1 To us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementOperatingActivitiesSegmentAxis" xlink:label="us-gaap_StatementOperatingActivitiesSegmentAxis_1" xlink:title="us-gaap_StatementOperatingActivitiesSegmentAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentOperatingActivitiesDomain" xlink:label="us-gaap_SegmentOperatingActivitiesDomain_1" xlink:title="us-gaap_SegmentOperatingActivitiesDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_StatementOperatingActivitiesSegmentAxis_1" xlink:to="us-gaap_SegmentOperatingActivitiesDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_StatementOperatingActivitiesSegmentAxis_1 To us-gaap_SegmentOperatingActivitiesDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="us-gaap_DerivativeInstrumentRiskAxis_1" xlink:title="us-gaap_DerivativeInstrumentRiskAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain_1" xlink:title="us-gaap_DerivativeContractTypeDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DerivativeInstrumentRiskAxis_1" xlink:to="us-gaap_DerivativeContractTypeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_DerivativeInstrumentRiskAxis_1 To us-gaap_DerivativeContractTypeDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis" xlink:label="us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_1" xlink:title="us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_HedgingRelationshipDomain" xlink:label="us-gaap_HedgingRelationshipDomain_1" xlink:title="us-gaap_HedgingRelationshipDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_1" xlink:to="us-gaap_HedgingRelationshipDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_1 To us-gaap_HedgingRelationshipDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:label="us-gaap_IncomeTaxAuthorityAxis_1" xlink:title="us-gaap_IncomeTaxAuthorityAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="us-gaap_IncomeTaxAuthorityDomain_1" xlink:title="us-gaap_IncomeTaxAuthorityDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_IncomeTaxAuthorityAxis_1" xlink:to="us-gaap_IncomeTaxAuthorityDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_IncomeTaxAuthorityAxis_1 To us-gaap_IncomeTaxAuthorityDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationAllowanceByDeferredTaxAssetAxis" xlink:label="us-gaap_ValuationAllowanceByDeferredTaxAssetAxis_1" xlink:title="us-gaap_ValuationAllowanceByDeferredTaxAssetAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetDomain" xlink:label="us-gaap_DeferredTaxAssetDomain_1" xlink:title="us-gaap_DeferredTaxAssetDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_ValuationAllowanceByDeferredTaxAssetAxis_1" xlink:to="us-gaap_DeferredTaxAssetDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_ValuationAllowanceByDeferredTaxAssetAxis_1 To us-gaap_DeferredTaxAssetDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:label="us-gaap_ConcentrationRiskByBenchmarkAxis_1" xlink:title="us-gaap_ConcentrationRiskByBenchmarkAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="us-gaap_ConcentrationRiskBenchmarkDomain_1" xlink:title="us-gaap_ConcentrationRiskBenchmarkDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_ConcentrationRiskByBenchmarkAxis_1" xlink:to="us-gaap_ConcentrationRiskBenchmarkDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_ConcentrationRiskByBenchmarkAxis_1 To us-gaap_ConcentrationRiskBenchmarkDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementAxis" xlink:label="srt_RestatementAxis_1" xlink:title="srt_RestatementAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementDomain" xlink:label="srt_RestatementDomain_1" xlink:title="srt_RestatementDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_RestatementAxis_1" xlink:to="srt_RestatementDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_RestatementAxis_1 To srt_RestatementDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionAxis" xlink:label="us-gaap_AssetAcquisitionAxis_1" xlink:title="us-gaap_AssetAcquisitionAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionDomain" xlink:label="us-gaap_AssetAcquisitionDomain_1" xlink:title="us-gaap_AssetAcquisitionDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_AssetAcquisitionAxis_1" xlink:to="us-gaap_AssetAcquisitionDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_AssetAcquisitionAxis_1 To us-gaap_AssetAcquisitionDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxPeriodAxis" xlink:label="us-gaap_TaxPeriodAxis_1" xlink:title="us-gaap_TaxPeriodAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxPeriodDomain" xlink:label="us-gaap_TaxPeriodDomain_1" xlink:title="us-gaap_TaxPeriodDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_TaxPeriodAxis_1" xlink:to="us-gaap_TaxPeriodDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_TaxPeriodAxis_1 To us-gaap_TaxPeriodDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis" xlink:label="us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis_1" xlink:title="us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProjectMember" xlink:label="us-gaap_ProjectMember_1" xlink:title="us-gaap_ProjectMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis_1" xlink:to="us-gaap_ProjectMember_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis_1 To us-gaap_ProjectMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressesAddressTypeAxis" xlink:label="dei_EntityAddressesAddressTypeAxis_1" xlink:title="dei_EntityAddressesAddressTypeAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_AddressTypeDomain" xlink:label="dei_AddressTypeDomain_1" xlink:title="dei_AddressTypeDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="dei_EntityAddressesAddressTypeAxis_1" xlink:to="dei_AddressTypeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:dei_EntityAddressesAddressTypeAxis_1 To dei_AddressTypeDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsAxis" xlink:label="us-gaap_DividendsAxis_1" xlink:title="us-gaap_DividendsAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsDomain" xlink:label="us-gaap_DividendsDomain_1" xlink:title="us-gaap_DividendsDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DividendsAxis_1" xlink:to="us-gaap_DividendsDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_DividendsAxis_1 To us-gaap_DividendsDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MajorCustomersAxis" xlink:label="srt_MajorCustomersAxis_1" xlink:title="srt_MajorCustomersAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_NameOfMajorCustomerDomain" xlink:label="srt_NameOfMajorCustomerDomain_1" xlink:title="srt_NameOfMajorCustomerDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_MajorCustomersAxis_1" xlink:to="srt_NameOfMajorCustomerDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_MajorCustomersAxis_1 To srt_NameOfMajorCustomerDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis" xlink:label="srt_CounterpartyNameAxis_1" xlink:title="srt_CounterpartyNameAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="srt_RepurchaseAgreementCounterpartyNameDomain_1" xlink:title="srt_RepurchaseAgreementCounterpartyNameDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_CounterpartyNameAxis_1" xlink:to="srt_RepurchaseAgreementCounterpartyNameDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_CounterpartyNameAxis_1 To srt_RepurchaseAgreementCounterpartyNameDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PartnerTypeOfPartnersCapitalAccountAxis" xlink:label="us-gaap_PartnerTypeOfPartnersCapitalAccountAxis_1" xlink:title="us-gaap_PartnerTypeOfPartnersCapitalAccountAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PartnerTypeOfPartnersCapitalAccountNameDomain" xlink:label="us-gaap_PartnerTypeOfPartnersCapitalAccountNameDomain_1" xlink:title="us-gaap_PartnerTypeOfPartnersCapitalAccountNameDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_PartnerTypeOfPartnersCapitalAccountAxis_1" xlink:to="us-gaap_PartnerTypeOfPartnersCapitalAccountNameDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_PartnerTypeOfPartnersCapitalAccountAxis_1 To us-gaap_PartnerTypeOfPartnersCapitalAccountNameDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityListingsExchangeAxis" xlink:label="dei_EntityListingsExchangeAxis_1" xlink:title="dei_EntityListingsExchangeAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_ExchangeDomain" xlink:label="dei_ExchangeDomain_1" xlink:title="dei_ExchangeDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="dei_EntityListingsExchangeAxis_1" xlink:to="dei_ExchangeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:dei_EntityListingsExchangeAxis_1 To dei_ExchangeDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis" xlink:label="us-gaap_DebtInstrumentAxis_1" xlink:title="us-gaap_DebtInstrumentAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain_1" xlink:title="us-gaap_DebtInstrumentNameDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DebtInstrumentAxis_1" xlink:to="us-gaap_DebtInstrumentNameDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_DebtInstrumentAxis_1 To us-gaap_DebtInstrumentNameDomain_1" order="1" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="extended" xlink:title="Individuals">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd#ecd_AllIndividualsMember" xlink:label="ecd_AllIndividualsMember" xlink:title="ecd_AllIndividualsMember" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_JohnD.SchmitzMember" xlink:label="wttr_JohnD.SchmitzMember" xlink:title="wttr_JohnD.SchmitzMember" />
    <link:definitionArc xlink:type="arc" xlink:from="ecd_AllIndividualsMember" xlink:to="wttr_JohnD.SchmitzMember" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:ecd_AllIndividualsMember To wttr_JohnD.SchmitzMember" order="1" priority="0" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DocumentDocumentAndEntityInformation" xlink:type="extended" xlink:title="995200090 - Document - Document and Entity Information">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentInformationTable" xlink:label="dei_DocumentInformationTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="dei_DocumentInformationTable" xlink:to="us-gaap_StatementClassOfStockAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember" xlink:label="us-gaap_CommonClassAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassAMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember" xlink:label="us-gaap_CommonClassBMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassBMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityListingsExchangeAxis" xlink:label="dei_EntityListingsExchangeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="dei_DocumentInformationTable" xlink:to="dei_EntityListingsExchangeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_ExchangeDomain" xlink:label="dei_ExchangeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="dei_EntityListingsExchangeAxis" xlink:to="dei_ExchangeDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/exch/2025/exch-2025.xsd#exch_XNYS" xlink:label="exch_XNYS" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="dei_ExchangeDomain" xlink:to="exch_XNYS" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_NyseTexasInc.Member" xlink:label="wttr_NyseTexasInc.Member" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="dei_ExchangeDomain" xlink:to="wttr_NyseTexasInc.Member" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentInformationLineItems" xlink:label="dei_DocumentInformationLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentInformationTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentType" xlink:label="dei_DocumentType" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentType" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentQuarterlyReport" xlink:label="dei_DocumentQuarterlyReport" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentQuarterlyReport" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentPeriodEndDate" xlink:label="dei_DocumentPeriodEndDate" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentPeriodEndDate" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentTransitionReport" xlink:label="dei_DocumentTransitionReport" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentTransitionReport" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityFileNumber" xlink:label="dei_EntityFileNumber" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityFileNumber" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityRegistrantName" xlink:label="dei_EntityRegistrantName" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityRegistrantName" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityIncorporationStateCountryCode" xlink:label="dei_EntityIncorporationStateCountryCode" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityIncorporationStateCountryCode" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityTaxIdentificationNumber" xlink:label="dei_EntityTaxIdentificationNumber" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityTaxIdentificationNumber" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressAddressLine1" xlink:label="dei_EntityAddressAddressLine1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityAddressAddressLine1" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressCityOrTown" xlink:label="dei_EntityAddressCityOrTown" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityAddressCityOrTown" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressStateOrProvince" xlink:label="dei_EntityAddressStateOrProvince" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="12" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityAddressStateOrProvince" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressPostalZipCode" xlink:label="dei_EntityAddressPostalZipCode" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="13" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityAddressPostalZipCode" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_CityAreaCode" xlink:label="dei_CityAreaCode" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="14" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_CityAreaCode" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LocalPhoneNumber" xlink:label="dei_LocalPhoneNumber" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="15" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_LocalPhoneNumber" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_Security12bTitle" xlink:label="dei_Security12bTitle" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="16" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_Security12bTitle" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_TradingSymbol" xlink:label="dei_TradingSymbol" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="17" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_TradingSymbol" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_SecurityExchangeName" xlink:label="dei_SecurityExchangeName" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="18" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_SecurityExchangeName" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCurrentReportingStatus" xlink:label="dei_EntityCurrentReportingStatus" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="19" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityCurrentReportingStatus" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityInteractiveDataCurrent" xlink:label="dei_EntityInteractiveDataCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="20" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityInteractiveDataCurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityFilerCategory" xlink:label="dei_EntityFilerCategory" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="21" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityFilerCategory" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntitySmallBusiness" xlink:label="dei_EntitySmallBusiness" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="22" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntitySmallBusiness" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityEmergingGrowthCompany" xlink:label="dei_EntityEmergingGrowthCompany" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="23" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityEmergingGrowthCompany" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityShellCompany" xlink:label="dei_EntityShellCompany" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="24" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityShellCompany" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCommonStockSharesOutstanding" xlink:label="dei_EntityCommonStockSharesOutstanding" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="25" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityCommonStockSharesOutstanding" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCentralIndexKey" xlink:label="dei_EntityCentralIndexKey" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="26" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityCentralIndexKey" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_CurrentFiscalYearEndDate" xlink:label="dei_CurrentFiscalYearEndDate" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="27" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_CurrentFiscalYearEndDate" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentFiscalYearFocus" xlink:label="dei_DocumentFiscalYearFocus" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="28" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentFiscalYearFocus" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentFiscalPeriodFocus" xlink:label="dei_DocumentFiscalPeriodFocus" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="29" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentFiscalPeriodFocus" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_AmendmentFlag" xlink:label="dei_AmendmentFlag" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="30" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_AmendmentFlag" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/StatementConsolidatedBalanceSheets" xlink:type="extended" xlink:title="995200100 - Statement - CONSOLIDATED BALANCE SHEETS">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementClassOfStockAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember" xlink:label="us-gaap_CommonClassAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassAMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember" xlink:label="us-gaap_CommonClassBMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassBMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="us-gaap_RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonrelatedPartyMember" xlink:label="us-gaap_NonrelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_NonrelatedPartyMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="us-gaap_RelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_RelatedPartyMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract" xlink:label="us-gaap_AssetsAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AssetsAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrentAbstract" xlink:label="us-gaap_AssetsCurrentAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_AssetsCurrentAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="us-gaap_CashAndCashEquivalentsAtCarryingValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_AssetsCurrentAbstract" xlink:to="us-gaap_CashAndCashEquivalentsAtCarryingValue" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsReceivableNetCurrent" xlink:label="us-gaap_AccountsReceivableNetCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_AssetsCurrentAbstract" xlink:to="us-gaap_AccountsReceivableNetCurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryNet" xlink:label="us-gaap_InventoryNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_AssetsCurrentAbstract" xlink:to="us-gaap_InventoryNet" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_AssetsCurrentAbstract" xlink:to="us-gaap_PrepaidExpenseAndOtherAssetsCurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrent" xlink:label="us-gaap_AssetsCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_AssetsCurrentAbstract" xlink:to="us-gaap_AssetsCurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="us-gaap_PropertyPlantAndEquipmentGross" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_PropertyPlantAndEquipmentGross" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:label="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="us-gaap_PropertyPlantAndEquipmentNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_PropertyPlantAndEquipmentNet" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAsset" xlink:label="us-gaap_OperatingLeaseRightOfUseAsset" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_OperatingLeaseRightOfUseAsset" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill" xlink:label="us-gaap_Goodwill" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_Goodwill" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherIntangibleAssetsNet" xlink:label="us-gaap_OtherIntangibleAssetsNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_OtherIntangibleAssetsNet" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxAssetsNet" xlink:label="us-gaap_DeferredIncomeTaxAssetsNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_DeferredIncomeTaxAssetsNet" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestments" xlink:label="us-gaap_EquityMethodInvestments" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_EquityMethodInvestments" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssetsNoncurrent" xlink:label="us-gaap_OtherAssetsNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_OtherAssetsNoncurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Assets" xlink:label="us-gaap_Assets" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_Assets" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:label="us-gaap_LiabilitiesAndStockholdersEquityAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LiabilitiesAndStockholdersEquityAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrentAbstract" xlink:label="us-gaap_LiabilitiesCurrentAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_LiabilitiesCurrentAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableCurrent" xlink:label="us-gaap_AccountsPayableCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_AccountsPayableCurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccruedLiabilitiesCurrent" xlink:label="us-gaap_AccruedLiabilitiesCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_AccruedLiabilitiesCurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableAndOtherAccruedLiabilities" xlink:label="us-gaap_AccountsPayableAndOtherAccruedLiabilities" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_AccountsPayableAndOtherAccruedLiabilities" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeRelatedLiabilitiesCurrent" xlink:label="us-gaap_EmployeeRelatedLiabilitiesCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_EmployeeRelatedLiabilitiesCurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccruedInsuranceCurrent" xlink:label="us-gaap_AccruedInsuranceCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_AccruedInsuranceCurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SalesAndExciseTaxPayableCurrent" xlink:label="us-gaap_SalesAndExciseTaxPayableCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_SalesAndExciseTaxPayableCurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccruedLiabilitiesAndOtherLiabilities" xlink:label="us-gaap_AccruedLiabilitiesAndOtherLiabilities" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_AccruedLiabilitiesAndOtherLiabilities" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityCurrent" xlink:label="us-gaap_OperatingLeaseLiabilityCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_OperatingLeaseLiabilityCurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtCurrent" xlink:label="us-gaap_LongTermDebtCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_LongTermDebtCurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinanceLeaseLiabilityCurrent" xlink:label="us-gaap_FinanceLeaseLiabilityCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_FinanceLeaseLiabilityCurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrent" xlink:label="us-gaap_LiabilitiesCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_LiabilitiesCurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_TaxReceivableAgreementsLiabilitiesNoncurrent" xlink:label="wttr_TaxReceivableAgreementsLiabilitiesNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="wttr_TaxReceivableAgreementsLiabilitiesNoncurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent" xlink:label="us-gaap_OperatingLeaseLiabilityNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_OperatingLeaseLiabilityNoncurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtNoncurrent" xlink:label="us-gaap_LongTermDebtNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_LongTermDebtNoncurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherLiabilitiesNoncurrent" xlink:label="us-gaap_OtherLiabilitiesNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_OtherLiabilitiesNoncurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Liabilities" xlink:label="us-gaap_Liabilities" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_Liabilities" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommitmentsAndContingencies" xlink:label="us-gaap_CommitmentsAndContingencies" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_CommitmentsAndContingencies" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockValue" xlink:label="us-gaap_CommonStockValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_CommonStockValue" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockValue" xlink:label="us-gaap_PreferredStockValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_PreferredStockValue" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapital" xlink:label="us-gaap_AdditionalPaidInCapital" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_AdditionalPaidInCapital" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsAccumulatedDeficit" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_RetainedEarningsAccumulatedDeficit" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity" xlink:label="us-gaap_StockholdersEquity" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="12" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_StockholdersEquity" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MinorityInterest" xlink:label="us-gaap_MinorityInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="13" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_MinorityInterest" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:label="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="14" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquity" xlink:label="us-gaap_LiabilitiesAndStockholdersEquity" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="15" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_LiabilitiesAndStockholdersEquity" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/StatementConsolidatedBalanceSheetsParenthetical" xlink:type="extended" xlink:title="995200105 - Statement - CONSOLIDATED BALANCE SHEETS (Parenthetical)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementClassOfStockAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember" xlink:label="us-gaap_CommonClassAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassAMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember" xlink:label="us-gaap_CommonClassBMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassBMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent" xlink:label="us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockParOrStatedValuePerShare" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="us-gaap_CommonStockSharesAuthorized" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesAuthorized" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued" xlink:label="us-gaap_CommonStockSharesIssued" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesIssued" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="us-gaap_CommonStockSharesOutstanding" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesOutstanding" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockParOrStatedValuePerShare" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockParOrStatedValuePerShare" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="us-gaap_PreferredStockSharesAuthorized" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockSharesAuthorized" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesIssued" xlink:label="us-gaap_PreferredStockSharesIssued" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockSharesIssued" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:label="us-gaap_PreferredStockSharesOutstanding" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockSharesOutstanding" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/StatementConsolidatedStatementsOfOperations" xlink:type="extended" xlink:title="995200200 - Statement - CONSOLIDATED STATEMENTS OF OPERATIONS">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementClassOfStockAxis" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember" xlink:label="us-gaap_CommonClassAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassAMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis" xlink:label="srt_ProductOrServiceAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_StatementTable" xlink:to="srt_ProductOrServiceAxis" priority="2" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain" xlink:label="srt_ProductsAndServicesDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_ProductOrServiceAxis" xlink:to="srt_ProductsAndServicesDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_WaterServicesMember" xlink:label="wttr_WaterServicesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_ProductsAndServicesDomain" xlink:to="wttr_WaterServicesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_WaterInfrastructureMember" xlink:label="wttr_WaterInfrastructureMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_ProductsAndServicesDomain" xlink:to="wttr_WaterInfrastructureMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_ChemicalTechnologiesMember" xlink:label="wttr_ChemicalTechnologiesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="srt_ProductsAndServicesDomain" xlink:to="wttr_ChemicalTechnologiesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenuesAbstract" xlink:label="us-gaap_RevenuesAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_RevenuesAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_RevenuesAbstract" xlink:to="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostOfRevenueAbstract" xlink:label="us-gaap_CostOfRevenueAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CostOfRevenueAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostOfGoodsAndServicesSold" xlink:label="us-gaap_CostOfGoodsAndServicesSold" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_CostOfRevenueAbstract" xlink:to="us-gaap_CostOfGoodsAndServicesSold" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_CostOfGoodsAndServicesSoldDepreciationAmortizationAccretionTotal" xlink:label="wttr_CostOfGoodsAndServicesSoldDepreciationAmortizationAccretionTotal" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_CostOfRevenueAbstract" xlink:to="wttr_CostOfGoodsAndServicesSoldDepreciationAmortizationAccretionTotal" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostOfRevenue" xlink:label="us-gaap_CostOfRevenue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_CostOfRevenueAbstract" xlink:to="us-gaap_CostOfRevenue" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GrossProfit" xlink:label="us-gaap_GrossProfit" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_CostOfRevenueAbstract" xlink:to="us-gaap_GrossProfit" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingExpensesAbstract" xlink:label="us-gaap_OperatingExpensesAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingExpensesAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SellingGeneralAndAdministrativeExpense" xlink:label="us-gaap_SellingGeneralAndAdministrativeExpense" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_SellingGeneralAndAdministrativeExpense" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationAndAmortization" xlink:label="us-gaap_DepreciationAndAmortization" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_DepreciationAndAmortization" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ImpairmentOfLongLivedAssetsHeldForUse" xlink:label="us-gaap_ImpairmentOfLongLivedAssetsHeldForUse" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_ImpairmentOfLongLivedAssetsHeldForUse" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_LeaseAbandonmentCosts" xlink:label="wttr_LeaseAbandonmentCosts" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="wttr_LeaseAbandonmentCosts" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingExpenses" xlink:label="us-gaap_OperatingExpenses" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_OperatingExpenses" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingIncomeLoss" xlink:label="us-gaap_OperatingIncomeLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingIncomeLoss" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonoperatingIncomeExpenseAbstract" xlink:label="us-gaap_NonoperatingIncomeExpenseAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NonoperatingIncomeExpenseAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnDispositionOfAssets1" xlink:label="us-gaap_GainLossOnDispositionOfAssets1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_NonoperatingIncomeExpenseAbstract" xlink:to="us-gaap_GainLossOnDispositionOfAssets1" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseNonoperating" xlink:label="us-gaap_InterestExpenseNonoperating" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_NonoperatingIncomeExpenseAbstract" xlink:to="us-gaap_InterestExpenseNonoperating" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_RemeasurementGainLossOnPropertyPlantAndEquipment" xlink:label="wttr_RemeasurementGainLossOnPropertyPlantAndEquipment" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_NonoperatingIncomeExpenseAbstract" xlink:to="wttr_RemeasurementGainLossOnPropertyPlantAndEquipment" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNonoperatingIncomeExpense" xlink:label="us-gaap_OtherNonoperatingIncomeExpense" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_NonoperatingIncomeExpenseAbstract" xlink:to="us-gaap_OtherNonoperatingIncomeExpense" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments" xlink:label="us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="us-gaap_IncomeTaxExpenseBenefit" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxExpenseBenefit" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromEquityMethodInvestments" xlink:label="us-gaap_IncomeLossFromEquityMethodInvestments" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeLossFromEquityMethodInvestments" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfitLoss" xlink:label="us-gaap_ProfitLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ProfitLoss" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" xlink:label="us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="12" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetIncomeLoss" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareAbstract" xlink:label="us-gaap_EarningsPerShareAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="13" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EarningsPerShareAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasic" xlink:label="us-gaap_EarningsPerShareBasic" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_EarningsPerShareAbstract" xlink:to="us-gaap_EarningsPerShareBasic" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDiluted" xlink:label="us-gaap_EarningsPerShareDiluted" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_EarningsPerShareAbstract" xlink:to="us-gaap_EarningsPerShareDiluted" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/StatementConsolidatedStatementsOfChangesInEquity" xlink:type="extended" xlink:title="995200400 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementClassOfStockAxis" priority="2" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember" xlink:label="us-gaap_CommonClassAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassAMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockMember" xlink:label="us-gaap_RestrictedStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_RestrictedStockMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember" xlink:label="us-gaap_CommonClassBMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassBMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="us-gaap_StatementEquityComponentsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementEquityComponentsAxis" priority="2" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ParentMember" xlink:label="us-gaap_ParentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_ParentMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember" xlink:label="us-gaap_CommonStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ParentMember" xlink:to="us-gaap_CommonStockMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="us-gaap_AdditionalPaidInCapitalMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ParentMember" xlink:to="us-gaap_AdditionalPaidInCapitalMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember" xlink:label="us-gaap_RetainedEarningsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ParentMember" xlink:to="us-gaap_RetainedEarningsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncontrollingInterestMember" xlink:label="us-gaap_NoncontrollingInterestMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_NoncontrollingInterestMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:label="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:label="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharesOutstanding" xlink:label="us-gaap_SharesOutstanding" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_SharesOutstanding" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockIssuedDuringPeriodValueNewIssues" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:to="us-gaap_StockIssuedDuringPeriodSharesNewIssues" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross" xlink:label="us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross" xlink:label="us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross" xlink:to="us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:label="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:to="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_StockIssuedDuringPeriodValueStockOptionsExercisedCashless" xlink:label="wttr_StockIssuedDuringPeriodValueStockOptionsExercisedCashless" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="wttr_StockIssuedDuringPeriodValueStockOptionsExercisedCashless" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodCashless" xlink:label="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodCashless" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="wttr_StockIssuedDuringPeriodValueStockOptionsExercisedCashless" xlink:to="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodCashless" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue" xlink:label="us-gaap_StockRepurchasedAndRetiredDuringPeriodValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockRepurchasedAndRetiredDuringPeriodValue" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares" xlink:label="us-gaap_StockRepurchasedAndRetiredDuringPeriodShares" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_StockRepurchasedAndRetiredDuringPeriodValue" xlink:to="us-gaap_StockRepurchasedAndRetiredDuringPeriodShares" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures" xlink:label="us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardForfeited" xlink:label="us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardForfeited" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures" xlink:to="us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardForfeited" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_StockIssuedDuringPeriodValuePerformanceSharesAwardVested" xlink:label="wttr_StockIssuedDuringPeriodValuePerformanceSharesAwardVested" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="wttr_StockIssuedDuringPeriodValuePerformanceSharesAwardVested" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_StockIssuedDuringPeriodSharesPerformanceSharesAwardVested" xlink:label="wttr_StockIssuedDuringPeriodSharesPerformanceSharesAwardVested" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="wttr_StockIssuedDuringPeriodValuePerformanceSharesAwardVested" xlink:to="wttr_StockIssuedDuringPeriodSharesPerformanceSharesAwardVested" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncontrollingInterestIncreaseFromSaleOfParentEquityInterest" xlink:label="us-gaap_NoncontrollingInterestIncreaseFromSaleOfParentEquityInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_NoncontrollingInterestIncreaseFromSaleOfParentEquityInterest" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="12" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfitLoss" xlink:label="us-gaap_ProfitLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="13" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_ProfitLoss" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/StatementConsolidatedStatementsOfChangesInEquityParenthetical" xlink:type="extended" xlink:title="995200405 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (Parenthetical)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementClassOfStockAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember" xlink:label="us-gaap_CommonClassAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassAMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockMember" xlink:label="us-gaap_RestrictedStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_RestrictedStockMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember" xlink:label="us-gaap_CommonClassBMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassBMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockDividendsPerShareDeclared" xlink:label="us-gaap_CommonStockDividendsPerShareDeclared" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockDividendsPerShareDeclared" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureAcquisitionsTables" xlink:type="extended" xlink:title="99930303 - Disclosure - ACQUISITIONS (Tables)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="us-gaap_BusinessAcquisitionAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_BusinessAcquisitionAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_OmniEnvironmentalSolutionsMember" xlink:label="wttr_OmniEnvironmentalSolutionsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="wttr_OmniEnvironmentalSolutionsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_ScheduleOfBusinessCombinationAndAssetAcquisitionTableTextBlock" xlink:label="wttr_ScheduleOfBusinessCombinationAndAssetAcquisitionTableTextBlock" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_StatementLineItems" xlink:to="wttr_ScheduleOfBusinessCombinationAndAssetAcquisitionTableTextBlock" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRecognizedIdentifiedAssetsAcquiredAndLiabilitiesAssumedTableTextBlock" xlink:label="us-gaap_ScheduleOfRecognizedIdentifiedAssetsAcquiredAndLiabilitiesAssumedTableTextBlock" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfRecognizedIdentifiedAssetsAcquiredAndLiabilitiesAssumedTableTextBlock" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureEquityBasedCompensationTables" xlink:type="extended" xlink:title="99931003 - Disclosure - EQUITY-BASED COMPENSATION (Tables)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_AwardTypeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockMember" xlink:label="us-gaap_RestrictedStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_RestrictedStockMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock" xlink:label="us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_ScheduleOfShareBasedCompensationRestrictedStockActivityTableTextBlock" xlink:label="wttr_ScheduleOfShareBasedCompensationRestrictedStockActivityTableTextBlock" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_StatementLineItems" xlink:to="wttr_ScheduleOfShareBasedCompensationRestrictedStockActivityTableTextBlock" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_ShareBasedCompensationPerformanceSharesAwardTargetedUnitsActivity" xlink:label="wttr_ShareBasedCompensationPerformanceSharesAwardTargetedUnitsActivity" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_StatementLineItems" xlink:to="wttr_ShareBasedCompensationPerformanceSharesAwardTargetedUnitsActivity" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationPerformanceSharesAwardUnvestedActivityTableTextBlock" xlink:label="us-gaap_ShareBasedCompensationPerformanceSharesAwardUnvestedActivityTableTextBlock" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationPerformanceSharesAwardUnvestedActivityTableTextBlock" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureBusinessAndBasisOfPresentationDetails" xlink:type="extended" xlink:title="99940101 - Disclosure - BUSINESS AND BASIS OF PRESENTATION (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable" xlink:label="us-gaap_ScheduleOfStockByClassTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="us-gaap_SubsidiarySaleOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfStockByClassTable" xlink:to="us-gaap_SubsidiarySaleOfStockAxis" priority="2" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_SecondaryPublicOfferingMember" xlink:label="wttr_SecondaryPublicOfferingMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="wttr_SecondaryPublicOfferingMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_SecondaryPublicOfferingBaseOfferingMember" xlink:label="wttr_SecondaryPublicOfferingBaseOfferingMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="wttr_SecondaryPublicOfferingMember" xlink:to="wttr_SecondaryPublicOfferingBaseOfferingMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OverAllotmentOptionMember" xlink:label="us-gaap_OverAllotmentOptionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="wttr_SecondaryPublicOfferingMember" xlink:to="us-gaap_OverAllotmentOptionMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfStockByClassTable" xlink:to="us-gaap_StatementClassOfStockAxis" priority="2" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember" xlink:label="us-gaap_CommonClassAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassAMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember" xlink:label="us-gaap_CommonClassBMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassBMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfStockByClassTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_CommonStockOfferingAbstract" xlink:label="wttr_CommonStockOfferingAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_StatementLineItems" xlink:to="wttr_CommonStockOfferingAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="wttr_CommonStockOfferingAbstract" xlink:to="us-gaap_StockIssuedDuringPeriodSharesNewIssues" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharesIssuedPricePerShare" xlink:label="us-gaap_SharesIssuedPricePerShare" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="wttr_CommonStockOfferingAbstract" xlink:to="us-gaap_SharesIssuedPricePerShare" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_AggregateProceedsFromIssuanceOfCommonStock" xlink:label="wttr_AggregateProceedsFromIssuanceOfCommonStock" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="wttr_CommonStockOfferingAbstract" xlink:to="wttr_AggregateProceedsFromIssuanceOfCommonStock" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:label="us-gaap_ProceedsFromIssuanceOfCommonStock" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="wttr_CommonStockOfferingAbstract" xlink:to="us-gaap_ProceedsFromIssuanceOfCommonStock" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockParOrStatedValuePerShare" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_VotingRightsNumberOfVotesPerShare" xlink:label="wttr_VotingRightsNumberOfVotesPerShare" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_StatementLineItems" xlink:to="wttr_VotingRightsNumberOfVotesPerShare" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityNoteStockSplitConversionRatio1" xlink:label="us-gaap_StockholdersEquityNoteStockSplitConversionRatio1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockholdersEquityNoteStockSplitConversionRatio1" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureSignificantAccountingPoliciesEquityInvestmentsDetails" xlink:type="extended" xlink:title="99940205 - Disclosure - SIGNIFICANT ACCOUNTING POLICIES - Equity Investments (Details)">
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_SignificantAccountingPoliciesTable" xlink:label="wttr_SignificantAccountingPoliciesTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipAxis" xlink:label="srt_OwnershipAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="wttr_SignificantAccountingPoliciesTable" xlink:to="srt_OwnershipAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipDomain" xlink:label="srt_OwnershipDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_OwnershipAxis" xlink:to="srt_OwnershipDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_AvFarmsLpMember" xlink:label="wttr_AvFarmsLpMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_OwnershipDomain" xlink:to="wttr_AvFarmsLpMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_AvFarmsManagementLlcGeneralPartnerMember" xlink:label="wttr_AvFarmsManagementLlcGeneralPartnerMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_OwnershipDomain" xlink:to="wttr_AvFarmsManagementLlcGeneralPartnerMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PartnerTypeOfPartnersCapitalAccountAxis" xlink:label="us-gaap_PartnerTypeOfPartnersCapitalAccountAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="wttr_SignificantAccountingPoliciesTable" xlink:to="us-gaap_PartnerTypeOfPartnersCapitalAccountAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PartnerTypeOfPartnersCapitalAccountNameDomain" xlink:label="us-gaap_PartnerTypeOfPartnersCapitalAccountNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_PartnerTypeOfPartnersCapitalAccountAxis" xlink:to="us-gaap_PartnerTypeOfPartnersCapitalAccountNameDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_SelectWaterReuseLlcMember" xlink:label="wttr_SelectWaterReuseLlcMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_PartnerTypeOfPartnersCapitalAccountNameDomain" xlink:to="wttr_SelectWaterReuseLlcMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_CAndRolloverCompanyLlcMember" xlink:label="wttr_CAndRolloverCompanyLlcMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_PartnerTypeOfPartnersCapitalAccountNameDomain" xlink:to="wttr_CAndRolloverCompanyLlcMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_GenesesWaterL.p.Member" xlink:label="wttr_GenesesWaterL.p.Member" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_PartnerTypeOfPartnersCapitalAccountNameDomain" xlink:to="wttr_GenesesWaterL.p.Member" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_SignificantAccountingPoliciesLineItems" xlink:label="wttr_SignificantAccountingPoliciesLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="wttr_SignificantAccountingPoliciesLineItems" xlink:to="wttr_SignificantAccountingPoliciesTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_NumberOfEquityMethodInvestee" xlink:label="wttr_NumberOfEquityMethodInvestee" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="wttr_SignificantAccountingPoliciesLineItems" xlink:to="wttr_NumberOfEquityMethodInvestee" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LimitedLiabilityCompanyLLCOrLimitedPartnershipLPMembersOrLimitedPartnersOwnershipInterest" xlink:label="us-gaap_LimitedLiabilityCompanyLLCOrLimitedPartnershipLPMembersOrLimitedPartnersOwnershipInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="wttr_SignificantAccountingPoliciesLineItems" xlink:to="us-gaap_LimitedLiabilityCompanyLLCOrLimitedPartnershipLPMembersOrLimitedPartnersOwnershipInterest" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LimitedPartnersContributedCapital" xlink:label="us-gaap_LimitedPartnersContributedCapital" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="wttr_SignificantAccountingPoliciesLineItems" xlink:to="us-gaap_LimitedPartnersContributedCapital" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureSignificantAccountingPoliciesInvestmentsInUnconsolidatedDetails" xlink:type="extended" xlink:title="99940206 - Disclosure - SIGNIFICANT ACCOUNTING POLICIES - Investments in Unconsolidated (Details)">
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_SignificantAccountingPoliciesTable" xlink:label="wttr_SignificantAccountingPoliciesTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:label="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="wttr_SignificantAccountingPoliciesTable" xlink:to="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="srt_EquityMethodInvesteeNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="srt_EquityMethodInvesteeNameDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_InvestmentInJointVentureOneMember" xlink:label="wttr_InvestmentInJointVentureOneMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_EquityMethodInvesteeNameDomain" xlink:to="wttr_InvestmentInJointVentureOneMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_InvestmentInJointVentureTwoMember" xlink:label="wttr_InvestmentInJointVentureTwoMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_EquityMethodInvesteeNameDomain" xlink:to="wttr_InvestmentInJointVentureTwoMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_InvestmentInJointVentureThreeMember" xlink:label="wttr_InvestmentInJointVentureThreeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="srt_EquityMethodInvesteeNameDomain" xlink:to="wttr_InvestmentInJointVentureThreeMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_InvestmentInJointVentureFourMember" xlink:label="wttr_InvestmentInJointVentureFourMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="srt_EquityMethodInvesteeNameDomain" xlink:to="wttr_InvestmentInJointVentureFourMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="us-gaap_BalanceSheetLocationAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="wttr_SignificantAccountingPoliciesTable" xlink:to="us-gaap_BalanceSheetLocationAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="us-gaap_BalanceSheetLocationDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_BalanceSheetLocationAxis" xlink:to="us-gaap_BalanceSheetLocationDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNoncurrentAssetsMember" xlink:label="us-gaap_OtherNoncurrentAssetsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_BalanceSheetLocationDomain" xlink:to="us-gaap_OtherNoncurrentAssetsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="wttr_SignificantAccountingPoliciesTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain" xlink:label="us-gaap_SegmentDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_WaterInfrastructureMember" xlink:label="wttr_WaterInfrastructureMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_WaterInfrastructureMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_WaterServicesMember" xlink:label="wttr_WaterServicesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_WaterServicesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_SignificantAccountingPoliciesLineItems" xlink:label="wttr_SignificantAccountingPoliciesLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="wttr_SignificantAccountingPoliciesLineItems" xlink:to="wttr_SignificantAccountingPoliciesTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestments" xlink:label="us-gaap_EquityMethodInvestments" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="wttr_SignificantAccountingPoliciesLineItems" xlink:to="us-gaap_EquityMethodInvestments" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage" xlink:label="us-gaap_EquityMethodInvestmentOwnershipPercentage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="wttr_SignificantAccountingPoliciesLineItems" xlink:to="us-gaap_EquityMethodInvestmentOwnershipPercentage" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureAcquisitionsDetails" xlink:type="extended" xlink:title="99940301 - Disclosure - ACQUISITIONS (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionTable" xlink:label="us-gaap_AssetAcquisitionTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionAxis" xlink:label="us-gaap_AssetAcquisitionAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_AssetAcquisitionTable" xlink:to="us-gaap_AssetAcquisitionAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionDomain" xlink:label="us-gaap_AssetAcquisitionDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_AssetAcquisitionAxis" xlink:to="us-gaap_AssetAcquisitionDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_AssetAcquisitionMember" xlink:label="wttr_AssetAcquisitionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_AssetAcquisitionDomain" xlink:to="wttr_AssetAcquisitionMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="us-gaap_BusinessAcquisitionAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_AssetAcquisitionTable" xlink:to="us-gaap_BusinessAcquisitionAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_OmniEnvironmentalSolutionsMember" xlink:label="wttr_OmniEnvironmentalSolutionsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="wttr_OmniEnvironmentalSolutionsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="us-gaap_AssetAcquisitionTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain" xlink:label="us-gaap_SegmentDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_OneSmallerAssetAcquisitionMember" xlink:label="wttr_OneSmallerAssetAcquisitionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_OneSmallerAssetAcquisitionMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_OneAssetAcquisitionMember" xlink:label="wttr_OneAssetAcquisitionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_OneAssetAcquisitionMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_EightSmallerAssetAcquisitionsMember" xlink:label="wttr_EightSmallerAssetAcquisitionsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_EightSmallerAssetAcquisitionsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionLineItems" xlink:label="us-gaap_AssetAcquisitionLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_AssetAcquisitionLineItems" xlink:to="us-gaap_AssetAcquisitionTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionAbstract" xlink:label="us-gaap_AssetAcquisitionAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_AssetAcquisitionLineItems" xlink:to="us-gaap_AssetAcquisitionAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireProductiveAssets" xlink:label="us-gaap_PaymentsToAcquireProductiveAssets" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_AssetAcquisitionAbstract" xlink:to="us-gaap_PaymentsToAcquireProductiveAssets" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionConsiderationTransferred" xlink:label="us-gaap_AssetAcquisitionConsiderationTransferred" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_AssetAcquisitionAbstract" xlink:to="us-gaap_AssetAcquisitionConsiderationTransferred" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationsAbstract" xlink:label="us-gaap_BusinessCombinationsAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_AssetAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationsAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued" xlink:label="us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_BusinessCombinationsAbstract" xlink:to="us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireBusinessesGross" xlink:label="us-gaap_PaymentsToAcquireBusinessesGross" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_BusinessCombinationsAbstract" xlink:to="us-gaap_PaymentsToAcquireBusinessesGross" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationConsiderationTransferredOther1" xlink:label="us-gaap_BusinessCombinationConsiderationTransferredOther1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_BusinessCombinationsAbstract" xlink:to="us-gaap_BusinessCombinationConsiderationTransferredOther1" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable" xlink:label="us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_BusinessCombinationsAbstract" xlink:to="us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationConsiderationTransferred1" xlink:label="us-gaap_BusinessCombinationConsiderationTransferred1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_BusinessCombinationsAbstract" xlink:to="us-gaap_BusinessCombinationConsiderationTransferred1" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_PaymentToAcquireBusinessGrossAndProductiveAssets" xlink:label="wttr_PaymentToAcquireBusinessGrossAndProductiveAssets" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_AssetAcquisitionLineItems" xlink:to="wttr_PaymentToAcquireBusinessGrossAndProductiveAssets" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BusinessCombinationAndAssetAcquisitionConsiderationTransferred" xlink:label="wttr_BusinessCombinationAndAssetAcquisitionConsiderationTransferred" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_AssetAcquisitionLineItems" xlink:to="wttr_BusinessCombinationAndAssetAcquisitionConsiderationTransferred" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureAcquisitionsBusinessCombinationDetails" xlink:type="extended" xlink:title="99940302 - Disclosure - ACQUISITIONS - Business Combination (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain" xlink:label="us-gaap_SegmentDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_WaterServicesMember" xlink:label="wttr_WaterServicesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_WaterServicesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_WaterInfrastructureMember" xlink:label="wttr_WaterInfrastructureMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_WaterInfrastructureMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationAxis" xlink:label="us-gaap_DisposalGroupClassificationAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:to="us-gaap_DisposalGroupClassificationAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain" xlink:label="us-gaap_DisposalGroupClassificationDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_DisposalGroupClassificationAxis" xlink:to="us-gaap_DisposalGroupClassificationDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupHeldforsaleNotDiscontinuedOperationsMember" xlink:label="us-gaap_DisposalGroupHeldforsaleNotDiscontinuedOperationsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_DisposalGroupClassificationDomain" xlink:to="us-gaap_DisposalGroupHeldforsaleNotDiscontinuedOperationsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="us-gaap_BusinessAcquisitionAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:to="us-gaap_BusinessAcquisitionAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_OmniEnvironmentalSolutionsMember" xlink:label="wttr_OmniEnvironmentalSolutionsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="wttr_OmniEnvironmentalSolutionsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="4" xlink:from="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:to="srt_RangeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="us-gaap_BusinessAcquisitionLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BusinessCombinationRemainingOfftakeCapacityOfSolidsWasteLandfill" xlink:label="wttr_BusinessCombinationRemainingOfftakeCapacityOfSolidsWasteLandfill" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="wttr_BusinessCombinationRemainingOfftakeCapacityOfSolidsWasteLandfill" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BusinessCombinationNumberOfSaltWaterDisposalFacilitiesAcquired" xlink:label="wttr_BusinessCombinationNumberOfSaltWaterDisposalFacilitiesAcquired" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="wttr_BusinessCombinationNumberOfSaltWaterDisposalFacilitiesAcquired" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BusinessCombinationSaltWaterDisposalFacilityPermittedDisposalCapacityPerDay" xlink:label="wttr_BusinessCombinationSaltWaterDisposalFacilityPermittedDisposalCapacityPerDay" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="wttr_BusinessCombinationSaltWaterDisposalFacilityPermittedDisposalCapacityPerDay" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BusinessCombinationCommercialOilStorageTankFarmCapacityAcquired" xlink:label="wttr_BusinessCombinationCommercialOilStorageTankFarmCapacityAcquired" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="wttr_BusinessCombinationCommercialOilStorageTankFarmCapacityAcquired" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireBusinessesGross" xlink:label="us-gaap_PaymentsToAcquireBusinessesGross" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_PaymentsToAcquireBusinessesGross" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_PaymentsToAcquireBusinessesBaseConsideration" xlink:label="wttr_PaymentsToAcquireBusinessesBaseConsideration" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="wttr_PaymentsToAcquireBusinessesBaseConsideration" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_PaymentsToAcquireBusinessesConsiderationForRetainedNetWorkingCapital" xlink:label="wttr_PaymentsToAcquireBusinessesConsiderationForRetainedNetWorkingCapital" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="wttr_PaymentsToAcquireBusinessesConsiderationForRetainedNetWorkingCapital" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued" xlink:label="us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BusinessCombinationNumberOfSaltWaterDisposalFacilitiesTransferredAsPartOfAcquisition" xlink:label="wttr_BusinessCombinationNumberOfSaltWaterDisposalFacilitiesTransferredAsPartOfAcquisition" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="wttr_BusinessCombinationNumberOfSaltWaterDisposalFacilitiesTransferredAsPartOfAcquisition" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BusinessCombinationNumberOfFluidsHaulingEmployeesTransferred" xlink:label="wttr_BusinessCombinationNumberOfFluidsHaulingEmployeesTransferred" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="wttr_BusinessCombinationNumberOfFluidsHaulingEmployeesTransferred" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment" xlink:label="us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="12" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_PercentageOfRevenue" xlink:label="wttr_PercentageOfRevenue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="13" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="wttr_PercentageOfRevenue" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_RemeasurementGainLossOnPropertyPlantAndEquipment" xlink:label="wttr_RemeasurementGainLossOnPropertyPlantAndEquipment" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="14" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="wttr_RemeasurementGainLossOnPropertyPlantAndEquipment" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts" xlink:label="us-gaap_BusinessCombinationAcquisitionRelatedCosts" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="15" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationAcquisitionRelatedCosts" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureAcquisitionsConsiderationTransferredAndEstimatedFairValueOfIdentifiedAssetsLiabilitiesDetails" xlink:type="extended" xlink:title="99940303 - Disclosure - ACQUISITIONS - Consideration transferred and estimated fair value of identified assets/(liabilities) (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="us-gaap_BusinessAcquisitionAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:to="us-gaap_BusinessAcquisitionAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_OmniEnvironmentalSolutionsMember" xlink:label="wttr_OmniEnvironmentalSolutionsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="wttr_OmniEnvironmentalSolutionsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="us-gaap_BusinessAcquisitionLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationConsiderationTransferredAbstract" xlink:label="us-gaap_BusinessCombinationConsiderationTransferredAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationConsiderationTransferredAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireBusinessesGross" xlink:label="us-gaap_PaymentsToAcquireBusinessesGross" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_BusinessCombinationConsiderationTransferredAbstract" xlink:to="us-gaap_PaymentsToAcquireBusinessesGross" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable" xlink:label="us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_BusinessCombinationConsiderationTransferredAbstract" xlink:to="us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BusinessCombinationConsiderationTransferredPropertyPlantAndEquipment" xlink:label="wttr_BusinessCombinationConsiderationTransferredPropertyPlantAndEquipment" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_BusinessCombinationConsiderationTransferredAbstract" xlink:to="wttr_BusinessCombinationConsiderationTransferredPropertyPlantAndEquipment" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BusinessCombinationConsiderationTransferredAssetsTransferredOrLiabilitiesAssumedOther" xlink:label="wttr_BusinessCombinationConsiderationTransferredAssetsTransferredOrLiabilitiesAssumedOther" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_BusinessCombinationConsiderationTransferredAbstract" xlink:to="wttr_BusinessCombinationConsiderationTransferredAssetsTransferredOrLiabilitiesAssumedOther" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationConsiderationTransferred1" xlink:label="us-gaap_BusinessCombinationConsiderationTransferred1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_BusinessCombinationConsiderationTransferredAbstract" xlink:to="us-gaap_BusinessCombinationConsiderationTransferred1" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_BusinessCombinationConsiderationTransferredAbstract" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedWorkingCapital" xlink:label="wttr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedWorkingCapital" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract" xlink:to="wttr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedWorkingCapital" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesAssetRetirementObligation" xlink:label="wttr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesAssetRetirementObligation" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract" xlink:to="wttr_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesAssetRetirementObligation" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill" xlink:label="us-gaap_Goodwill" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract" xlink:to="us-gaap_Goodwill" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentFairValueAllocatedToNetAssets" xlink:label="wttr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentFairValueAllocatedToNetAssets" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract" xlink:to="wttr_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentFairValueAllocatedToNetAssets" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued" xlink:label="us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract" xlink:to="us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BusinessCombinationConsiderationTransferredPropertyPlantAndEquipmentCarryingValue" xlink:label="wttr_BusinessCombinationConsiderationTransferredPropertyPlantAndEquipmentCarryingValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract" xlink:to="wttr_BusinessCombinationConsiderationTransferredPropertyPlantAndEquipmentCarryingValue" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_RemeasurementGainLossOnPropertyPlantAndEquipment" xlink:label="wttr_RemeasurementGainLossOnPropertyPlantAndEquipment" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetsAbstract" xlink:to="wttr_RemeasurementGainLossOnPropertyPlantAndEquipment" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureAcquisitionsAssetAcquisitionsDetails" xlink:type="extended" xlink:title="99940304 - Disclosure - ACQUISITIONS - Asset Acquisitions (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionAxis" xlink:label="us-gaap_AssetAcquisitionAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:to="us-gaap_AssetAcquisitionAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionDomain" xlink:label="us-gaap_AssetAcquisitionDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_AssetAcquisitionAxis" xlink:to="us-gaap_AssetAcquisitionDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_AssetAcquisitionMember" xlink:label="wttr_AssetAcquisitionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_AssetAcquisitionDomain" xlink:to="wttr_AssetAcquisitionMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain" xlink:label="us-gaap_SegmentDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_EightSmallerAssetAcquisitionsMember" xlink:label="wttr_EightSmallerAssetAcquisitionsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_EightSmallerAssetAcquisitionsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_SevenSmallerAssetAcquisitionsMember" xlink:label="wttr_SevenSmallerAssetAcquisitionsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_SevenSmallerAssetAcquisitionsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_OneAssetAcquisitionMember" xlink:label="wttr_OneAssetAcquisitionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_OneAssetAcquisitionMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_OneSmallerAssetAcquisitionMember" xlink:label="wttr_OneSmallerAssetAcquisitionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_OneSmallerAssetAcquisitionMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="us-gaap_BusinessAcquisitionLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_AssetAcquisitionsNumberOfTransaction" xlink:label="wttr_AssetAcquisitionsNumberOfTransaction" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="wttr_AssetAcquisitionsNumberOfTransaction" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionConsiderationTransferred" xlink:label="us-gaap_AssetAcquisitionConsiderationTransferred" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_AssetAcquisitionConsiderationTransferred" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_AssetAcquisitionsAreaOfLand" xlink:label="wttr_AssetAcquisitionsAreaOfLand" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="wttr_AssetAcquisitionsAreaOfLand" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="us-gaap_PropertyPlantAndEquipmentNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentNet" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_AssetAcquisitionRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetRetirementObligationsAndOtherLiabilities" xlink:label="wttr_AssetAcquisitionRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetRetirementObligationsAndOtherLiabilities" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="wttr_AssetAcquisitionRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssetRetirementObligationsAndOtherLiabilities" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireBuildings" xlink:label="us-gaap_PaymentsToAcquireBuildings" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_PaymentsToAcquireBuildings" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureRevenueDetails" xlink:type="extended" xlink:title="99940401 - Disclosure - REVENUE (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable" xlink:label="us-gaap_DisaggregationOfRevenueTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis" xlink:label="srt_StatementGeographicalAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_DisaggregationOfRevenueTable" xlink:to="srt_StatementGeographicalAxis" priority="2" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain" xlink:label="srt_SegmentGeographicalDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_StatementGeographicalAxis" xlink:to="srt_SegmentGeographicalDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_PermianBasinMember" xlink:label="wttr_PermianBasinMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_SegmentGeographicalDomain" xlink:to="wttr_PermianBasinMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_EagleFordMember" xlink:label="wttr_EagleFordMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_SegmentGeographicalDomain" xlink:to="wttr_EagleFordMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_RockiesMember" xlink:label="wttr_RockiesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="srt_SegmentGeographicalDomain" xlink:to="wttr_RockiesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_MarcellusUticaMember" xlink:label="wttr_MarcellusUticaMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="srt_SegmentGeographicalDomain" xlink:to="wttr_MarcellusUticaMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BakkenMember" xlink:label="wttr_BakkenMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="srt_SegmentGeographicalDomain" xlink:to="wttr_BakkenMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_MidconMember" xlink:label="wttr_MidconMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="srt_SegmentGeographicalDomain" xlink:to="wttr_MidconMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_HaynesvilleE.TexasMember" xlink:label="wttr_HaynesvilleE.TexasMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="srt_SegmentGeographicalDomain" xlink:to="wttr_HaynesvilleE.TexasMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_EliminationsAndOtherGeographicalRegionsMember" xlink:label="wttr_EliminationsAndOtherGeographicalRegionsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="srt_SegmentGeographicalDomain" xlink:to="wttr_EliminationsAndOtherGeographicalRegionsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_PermianBasinHaynesvilleAndBakkenMember" xlink:label="wttr_PermianBasinHaynesvilleAndBakkenMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="srt_SegmentGeographicalDomain" xlink:to="wttr_PermianBasinHaynesvilleAndBakkenMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_PermianBasinMarcellusUticaAndRockiesMember" xlink:label="wttr_PermianBasinMarcellusUticaAndRockiesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="srt_SegmentGeographicalDomain" xlink:to="wttr_PermianBasinMarcellusUticaAndRockiesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_PermianBasinMidconAndHaynesvilleMember" xlink:label="wttr_PermianBasinMidconAndHaynesvilleMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="srt_SegmentGeographicalDomain" xlink:to="wttr_PermianBasinMidconAndHaynesvilleMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_DisaggregationOfRevenueTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" priority="2" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain" xlink:label="us-gaap_SegmentDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_WaterServicesMember" xlink:label="wttr_WaterServicesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_WaterServicesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_WaterInfrastructureMember" xlink:label="wttr_WaterInfrastructureMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_WaterInfrastructureMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_OilfieldChemicalsMember" xlink:label="wttr_OilfieldChemicalsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_OilfieldChemicalsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis" xlink:label="srt_ProductOrServiceAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="us-gaap_DisaggregationOfRevenueTable" xlink:to="srt_ProductOrServiceAxis" priority="2" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain" xlink:label="srt_ProductsAndServicesDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_ProductOrServiceAxis" xlink:to="srt_ProductsAndServicesDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_AccommodationsAndRentalsRevenueMember" xlink:label="wttr_AccommodationsAndRentalsRevenueMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_ProductsAndServicesDomain" xlink:to="wttr_AccommodationsAndRentalsRevenueMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:label="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="4" xlink:from="us-gaap_DisaggregationOfRevenueTable" xlink:to="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" priority="2" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="us-gaap_TypeOfAdoptionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:to="us-gaap_TypeOfAdoptionMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountingStandardsUpdate201602Member" xlink:label="us-gaap_AccountingStandardsUpdate201602Member" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_TypeOfAdoptionMember" xlink:to="us-gaap_AccountingStandardsUpdate201602Member" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems" xlink:label="us-gaap_DisaggregationOfRevenueLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_DisaggregationOfRevenueLineItems" xlink:to="us-gaap_DisaggregationOfRevenueTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiability" xlink:label="us-gaap_ContractWithCustomerLiability" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_DisaggregationOfRevenueLineItems" xlink:to="us-gaap_ContractWithCustomerLiability" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized" xlink:label="us-gaap_ContractWithCustomerLiabilityRevenueRecognized" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_DisaggregationOfRevenueLineItems" xlink:to="us-gaap_ContractWithCustomerLiabilityRevenueRecognized" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_DisaggregationOfRevenueLineItems" xlink:to="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MachineryAndEquipmentGross" xlink:label="us-gaap_MachineryAndEquipmentGross" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_DisaggregationOfRevenueLineItems" xlink:to="us-gaap_MachineryAndEquipmentGross" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_NumberOfRevenueProducingRegions" xlink:label="wttr_NumberOfRevenueProducingRegions" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_DisaggregationOfRevenueLineItems" xlink:to="wttr_NumberOfRevenueProducingRegions" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_PercentageOfRevenue" xlink:label="wttr_PercentageOfRevenue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_DisaggregationOfRevenueLineItems" xlink:to="wttr_PercentageOfRevenue" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureInventoriesDetails" xlink:type="extended" xlink:title="99940501 - Disclosure - INVENTORIES (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryCurrentTable" xlink:label="us-gaap_InventoryCurrentTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_InventoryCurrentTable" xlink:to="srt_RangeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryLineItems" xlink:label="us-gaap_InventoryLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_InventoryLineItems" xlink:to="us-gaap_InventoryCurrentTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryNetAbstract" xlink:label="us-gaap_InventoryNetAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_InventoryLineItems" xlink:to="us-gaap_InventoryNetAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryRawMaterials" xlink:label="us-gaap_InventoryRawMaterials" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_InventoryNetAbstract" xlink:to="us-gaap_InventoryRawMaterials" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryFinishedGoods" xlink:label="us-gaap_InventoryFinishedGoods" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_InventoryNetAbstract" xlink:to="us-gaap_InventoryFinishedGoods" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryNet" xlink:label="us-gaap_InventoryNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_InventoryNetAbstract" xlink:to="us-gaap_InventoryNet" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryWriteDown" xlink:label="us-gaap_InventoryWriteDown" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_InventoryLineItems" xlink:to="us-gaap_InventoryWriteDown" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryValuationReserves" xlink:label="us-gaap_InventoryValuationReserves" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_InventoryLineItems" xlink:to="us-gaap_InventoryValuationReserves" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosurePropertyAndEquipmentDetails" xlink:type="extended" xlink:title="99940601 - Disclosure - PROPERTY AND EQUIPMENT (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="us-gaap_PropertyPlantAndEquipmentByTypeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MachineryAndEquipmentMember" xlink:label="us-gaap_MachineryAndEquipmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_MachineryAndEquipmentMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BuildingAndBuildingImprovementsMember" xlink:label="us-gaap_BuildingAndBuildingImprovementsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_BuildingAndBuildingImprovementsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WellsAndRelatedEquipmentAndFacilitiesMember" xlink:label="us-gaap_WellsAndRelatedEquipmentAndFacilitiesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_WellsAndRelatedEquipmentAndFacilitiesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_RecyclingFacilitiesMember" xlink:label="wttr_RecyclingFacilitiesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="wttr_RecyclingFacilitiesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PipelinesMember" xlink:label="us-gaap_PipelinesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_PipelinesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VehiclesMember" xlink:label="us-gaap_VehiclesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_VehiclesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LandMember" xlink:label="us-gaap_LandMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_LandMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComputerEquipmentMember" xlink:label="us-gaap_ComputerEquipmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_ComputerEquipmentMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OfficeEquipmentMember" xlink:label="us-gaap_OfficeEquipmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_OfficeEquipmentMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_FinanceLeasedMachineryAndEquipmentMember" xlink:label="wttr_FinanceLeasedMachineryAndEquipmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="wttr_FinanceLeasedMachineryAndEquipmentMember" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_FinanceLeasedComputerEquipmentMember" xlink:label="wttr_FinanceLeasedComputerEquipmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="wttr_FinanceLeasedComputerEquipmentMember" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConstructionInProgressMember" xlink:label="us-gaap_ConstructionInProgressMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="12" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_ConstructionInProgressMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain" xlink:label="us-gaap_SegmentDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_WaterInfrastructureMember" xlink:label="wttr_WaterInfrastructureMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_WaterInfrastructureMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_WaterServicesMember" xlink:label="wttr_WaterServicesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_WaterServicesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="us-gaap_PropertyPlantAndEquipmentLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="us-gaap_PropertyPlantAndEquipmentGross" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentGross" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:label="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="us-gaap_PropertyPlantAndEquipmentNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentNet" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinanceLeaseRightOfUseAssetAccumulatedAmortization" xlink:label="us-gaap_FinanceLeaseRightOfUseAssetAccumulatedAmortization" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_FinanceLeaseRightOfUseAssetAccumulatedAmortization" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ImpairmentOfLongLivedAssetsHeldForUse" xlink:label="us-gaap_ImpairmentOfLongLivedAssetsHeldForUse" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_ImpairmentOfLongLivedAssetsHeldForUse" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationAbstract" xlink:label="us-gaap_DepreciationAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_DepreciationAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Depreciation" xlink:label="us-gaap_Depreciation" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_DepreciationAbstract" xlink:to="us-gaap_Depreciation" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinanceLeaseRightOfUseAssetAmortization" xlink:label="us-gaap_FinanceLeaseRightOfUseAssetAmortization" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_DepreciationAbstract" xlink:to="us-gaap_FinanceLeaseRightOfUseAssetAmortization" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfIntangibleAssets" xlink:label="us-gaap_AmortizationOfIntangibleAssets" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_DepreciationAbstract" xlink:to="us-gaap_AmortizationOfIntangibleAssets" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccretionExpenseIncludingAssetRetirementObligations" xlink:label="us-gaap_AccretionExpenseIncludingAssetRetirementObligations" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_DepreciationAbstract" xlink:to="us-gaap_AccretionExpenseIncludingAssetRetirementObligations" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationAmortizationAndAccretionNet" xlink:label="us-gaap_DepreciationAmortizationAndAccretionNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_DepreciationAbstract" xlink:to="us-gaap_DepreciationAmortizationAndAccretionNet" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureGoodwillAndOtherIntangibleAssetsChangesInCarryingAmountsOfGoodwillDetails" xlink:type="extended" xlink:title="99940701 - Disclosure - GOODWILL AND OTHER INTANGIBLE ASSETS - Changes in the Carrying Amounts of Goodwill (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfGoodwillTable" xlink:label="us-gaap_ScheduleOfGoodwillTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfGoodwillTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain" xlink:label="us-gaap_SegmentDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_WaterServicesMember" xlink:label="wttr_WaterServicesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_WaterServicesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_WaterInfrastructureMember" xlink:label="wttr_WaterInfrastructureMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_WaterInfrastructureMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillLineItems" xlink:label="us-gaap_GoodwillLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_GoodwillLineItems" xlink:to="us-gaap_ScheduleOfGoodwillTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillPeriodIncreaseDecrease" xlink:label="us-gaap_GoodwillPeriodIncreaseDecrease" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_GoodwillLineItems" xlink:to="us-gaap_GoodwillPeriodIncreaseDecrease" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillRollForward" xlink:label="us-gaap_GoodwillRollForward" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_GoodwillLineItems" xlink:to="us-gaap_GoodwillRollForward" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill" xlink:label="us-gaap_Goodwill" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_GoodwillRollForward" xlink:to="us-gaap_Goodwill" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillAcquiredDuringPeriod" xlink:label="us-gaap_GoodwillAcquiredDuringPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_GoodwillRollForward" xlink:to="us-gaap_GoodwillAcquiredDuringPeriod" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureGoodwillAndOtherIntangibleAssetsOtherIntangibleAssetsDetails" xlink:type="extended" xlink:title="99940702 - Disclosure - GOODWILL AND OTHER INTANGIBLE ASSETS - Other Intangible Assets (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:label="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis" priority="2" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WaterPlantMember" xlink:label="us-gaap_WaterPlantMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_WaterPlantMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" priority="2" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerRelationshipsMember" xlink:label="us-gaap_CustomerRelationshipsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_CustomerRelationshipsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PatentsMember" xlink:label="us-gaap_PatentsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_PatentsMember" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_WaterPlantMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:label="us-gaap_FiniteLivedIntangibleAssetsLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsGross" xlink:label="us-gaap_FiniteLivedIntangibleAssetsGross" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsGross" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsNet" xlink:label="us-gaap_FiniteLivedIntangibleAssetsNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsNet" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill" xlink:label="us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_IntangibleAssetsIndefiniteLivedExcludingGoodwillNet" xlink:label="wttr_IntangibleAssetsIndefiniteLivedExcludingGoodwillNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="wttr_IntangibleAssetsIndefiniteLivedExcludingGoodwillNet" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IntangibleAssetsGrossExcludingGoodwill" xlink:label="us-gaap_IntangibleAssetsGrossExcludingGoodwill" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_IntangibleAssetsGrossExcludingGoodwill" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill" xlink:label="us-gaap_IntangibleAssetsNetExcludingGoodwill" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_IntangibleAssetsNetExcludingGoodwill" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureGoodwillAndOtherIntangibleAssetsOtherIntangibleAssetsNarrativeDetails" xlink:type="extended" xlink:title="99940703 - Disclosure - GOODWILL AND OTHER INTANGIBLE ASSETS - Other Intangible Assets Narrative (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:label="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" priority="2" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerRelationshipsMember" xlink:label="us-gaap_CustomerRelationshipsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_CustomerRelationshipsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PatentsMember" xlink:label="us-gaap_PatentsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_PatentsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WaterPlantMember" xlink:label="us-gaap_WaterPlantMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_WaterPlantMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_WaterPlantMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="us-gaap_BusinessAcquisitionAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="us-gaap_BusinessAcquisitionAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_TotalAcquisitionsMember" xlink:label="wttr_TotalAcquisitionsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="wttr_TotalAcquisitionsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="4" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="srt_RangeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:label="us-gaap_FiniteLivedIntangibleAssetsLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife" xlink:label="us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_WeightedAverageRemainingAmortizationPeriod" xlink:label="wttr_WeightedAverageRemainingAmortizationPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="wttr_WeightedAverageRemainingAmortizationPeriod" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_IndefiniteLivedIntangibleAssetWeightedAveragePeriodRenewalTerm" xlink:label="wttr_IndefiniteLivedIntangibleAssetWeightedAveragePeriodRenewalTerm" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="wttr_IndefiniteLivedIntangibleAssetWeightedAveragePeriodRenewalTerm" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureDebtDetails" xlink:type="extended" xlink:title="99940801 - Disclosure - DEBT (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable" xlink:label="us-gaap_DebtInstrumentTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis" xlink:label="us-gaap_CreditFacilityAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_CreditFacilityAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain" xlink:label="us-gaap_CreditFacilityDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_CreditFacilityAxis" xlink:to="us-gaap_CreditFacilityDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevolvingCreditFacilityMember" xlink:label="us-gaap_RevolvingCreditFacilityMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_CreditFacilityDomain" xlink:to="us-gaap_RevolvingCreditFacilityMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_SeniorSecuredCreditFacilityMember" xlink:label="wttr_SeniorSecuredCreditFacilityMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_CreditFacilityDomain" xlink:to="wttr_SeniorSecuredCreditFacilityMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_TermLoanFacilityMember" xlink:label="wttr_TermLoanFacilityMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_CreditFacilityDomain" xlink:to="wttr_TermLoanFacilityMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_OldSustainabilityLinkedCreditFacilityMember" xlink:label="wttr_OldSustainabilityLinkedCreditFacilityMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_CreditFacilityDomain" xlink:to="wttr_OldSustainabilityLinkedCreditFacilityMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeAxis" xlink:label="us-gaap_ShortTermDebtTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_ShortTermDebtTypeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeDomain" xlink:label="us-gaap_ShortTermDebtTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_ShortTermDebtTypeAxis" xlink:to="us-gaap_ShortTermDebtTypeDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LetterOfCreditMember" xlink:label="us-gaap_LetterOfCreditMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShortTermDebtTypeDomain" xlink:to="us-gaap_LetterOfCreditMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BridgeLoanMember" xlink:label="us-gaap_BridgeLoanMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShortTermDebtTypeDomain" xlink:to="us-gaap_BridgeLoanMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementScenarioAxis" xlink:label="srt_StatementScenarioAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="srt_StatementScenarioAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="srt_ScenarioUnspecifiedDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_StatementScenarioAxis" xlink:to="srt_ScenarioUnspecifiedDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_FirstFourYearsMember" xlink:label="wttr_FirstFourYearsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_ScenarioUnspecifiedDomain" xlink:to="wttr_FirstFourYearsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BorrowingBaseAxis" xlink:label="wttr_BorrowingBaseAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="4" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="wttr_BorrowingBaseAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BorrowingBaseDomain" xlink:label="wttr_BorrowingBaseDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="wttr_BorrowingBaseAxis" xlink:to="wttr_BorrowingBaseDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_EligibleBilledReceivablesMember" xlink:label="wttr_EligibleBilledReceivablesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="wttr_BorrowingBaseDomain" xlink:to="wttr_EligibleBilledReceivablesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_EligibleUnbilledReceivablesMember" xlink:label="wttr_EligibleUnbilledReceivablesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="wttr_BorrowingBaseDomain" xlink:to="wttr_EligibleUnbilledReceivablesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_EligibleInventoryMember" xlink:label="wttr_EligibleInventoryMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="wttr_BorrowingBaseDomain" xlink:to="wttr_EligibleInventoryMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_NetRecoveryPercentageMember" xlink:label="wttr_NetRecoveryPercentageMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="wttr_BorrowingBaseDomain" xlink:to="wttr_NetRecoveryPercentageMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_EligibleInvestmentGradeBilledReceivablesMember" xlink:label="wttr_EligibleInvestmentGradeBilledReceivablesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="wttr_BorrowingBaseDomain" xlink:to="wttr_EligibleInvestmentGradeBilledReceivablesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BorrowingBaseMember" xlink:label="wttr_BorrowingBaseMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="wttr_BorrowingBaseDomain" xlink:to="wttr_BorrowingBaseMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_NetBookValueOfAllMEMember" xlink:label="wttr_NetBookValueOfAllMEMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="wttr_BorrowingBaseDomain" xlink:to="wttr_NetBookValueOfAllMEMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_NolvIpOfTermAdvanceCollateralMember" xlink:label="wttr_NolvIpOfTermAdvanceCollateralMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="wttr_BorrowingBaseDomain" xlink:to="wttr_NolvIpOfTermAdvanceCollateralMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateAxis" xlink:label="us-gaap_VariableRateAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="5" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_VariableRateAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain" xlink:label="us-gaap_VariableRateDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_VariableRateAxis" xlink:to="us-gaap_VariableRateDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BaseRateMember" xlink:label="us-gaap_BaseRateMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_VariableRateDomain" xlink:to="us-gaap_BaseRateMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredOvernightFinancingRateSofrMember" xlink:label="us-gaap_SecuredOvernightFinancingRateSofrMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_VariableRateDomain" xlink:to="us-gaap_SecuredOvernightFinancingRateSofrMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalFundsEffectiveSwapRateMember" xlink:label="us-gaap_FederalFundsEffectiveSwapRateMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_VariableRateDomain" xlink:to="us-gaap_FederalFundsEffectiveSwapRateMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="6" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="srt_RangeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="us-gaap_DebtInstrumentLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity" xlink:label="us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_LineOfCreditFacilityContingentIncreaseAdditionalBorrowingCapacity" xlink:label="wttr_LineOfCreditFacilityContingentIncreaseAdditionalBorrowingCapacity" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="wttr_LineOfCreditFacilityContingentIncreaseAdditionalBorrowingCapacity" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_CollateralPercentOfAsset" xlink:label="wttr_CollateralPercentOfAsset" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="wttr_CollateralPercentOfAsset" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_CollateralBorrowingBasePercentOne" xlink:label="wttr_CollateralBorrowingBasePercentOne" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="wttr_CollateralBorrowingBasePercentOne" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_CollateralBorrowingBasePercentTwo" xlink:label="wttr_CollateralBorrowingBasePercentTwo" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="wttr_CollateralBorrowingBasePercentTwo" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_LineOfCreditFacilityBorrowingBase" xlink:label="wttr_LineOfCreditFacilityBorrowingBase" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="wttr_LineOfCreditFacilityBorrowingBase" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_LineOfCreditFacilityBorrowingLimitPercentage" xlink:label="wttr_LineOfCreditFacilityBorrowingLimitPercentage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="wttr_LineOfCreditFacilityBorrowingLimitPercentage" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_LineOfCreditFacilityBorrowingLimit" xlink:label="wttr_LineOfCreditFacilityBorrowingLimit" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="wttr_LineOfCreditFacilityBorrowingLimit" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_LineOfCreditFacilityLookBackPeriodRedeterminationBeginning" xlink:label="wttr_LineOfCreditFacilityLookBackPeriodRedeterminationBeginning" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="wttr_LineOfCreditFacilityLookBackPeriodRedeterminationBeginning" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_LineOfCreditFacilityLookBackPeriodRedeterminationEnding" xlink:label="wttr_LineOfCreditFacilityLookBackPeriodRedeterminationEnding" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="wttr_LineOfCreditFacilityLookBackPeriodRedeterminationEnding" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_DebtInstrumentFloorRate" xlink:label="wttr_DebtInstrumentFloorRate" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="12" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="wttr_DebtInstrumentFloorRate" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" xlink:label="us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="13" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtPercentageBearingVariableInterestRate" xlink:label="us-gaap_LongTermDebtPercentageBearingVariableInterestRate" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="14" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LongTermDebtPercentageBearingVariableInterestRate" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage" xlink:label="us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="15" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_FixedChargeCoverageRatio" xlink:label="wttr_FixedChargeCoverageRatio" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="16" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="wttr_FixedChargeCoverageRatio" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_LeverageRatio" xlink:label="wttr_LeverageRatio" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="17" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="wttr_LeverageRatio" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_LineOfCreditFacilityLookBackPeriodForDistributions" xlink:label="wttr_LineOfCreditFacilityLookBackPeriodForDistributions" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="18" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="wttr_LineOfCreditFacilityLookBackPeriodForDistributions" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_LineOfCreditFacilityPercentageOutstanding" xlink:label="wttr_LineOfCreditFacilityPercentageOutstanding" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="19" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="wttr_LineOfCreditFacilityPercentageOutstanding" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_LineOfCreditFacilityBaseValueForDistributions" xlink:label="wttr_LineOfCreditFacilityBaseValueForDistributions" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="20" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="wttr_LineOfCreditFacilityBaseValueForDistributions" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityPeriodicPayment" xlink:label="us-gaap_LineOfCreditFacilityPeriodicPayment" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="21" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LineOfCreditFacilityPeriodicPayment" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebt" xlink:label="us-gaap_LongTermDebt" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="22" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LongTermDebt" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_LineOfCreditFacilityReductionInMaximumBorrowingCapacity" xlink:label="wttr_LineOfCreditFacilityReductionInMaximumBorrowingCapacity" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="23" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="wttr_LineOfCreditFacilityReductionInMaximumBorrowingCapacity" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity" xlink:label="us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="24" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsOfDebtIssuanceCosts" xlink:label="us-gaap_PaymentsOfDebtIssuanceCosts" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="25" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_PaymentsOfDebtIssuanceCosts" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnamortizedDebtIssuanceExpense" xlink:label="us-gaap_UnamortizedDebtIssuanceExpense" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="26" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_UnamortizedDebtIssuanceExpense" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfFinancingCosts" xlink:label="us-gaap_AmortizationOfFinancingCosts" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="27" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_AmortizationOfFinancingCosts" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_UnamortizedDebtIssuanceCostsTransferred" xlink:label="wttr_UnamortizedDebtIssuanceCostsTransferred" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="28" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="wttr_UnamortizedDebtIssuanceCostsTransferred" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureDebtPrincipalMaturitiesOfDebtOutstandingDetails" xlink:type="extended" xlink:title="99940802 - Disclosure - DEBT - Principal Maturities Of Debt Outstanding (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable" xlink:label="us-gaap_DebtInstrumentTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis" xlink:label="us-gaap_CreditFacilityAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_CreditFacilityAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain" xlink:label="us-gaap_CreditFacilityDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_CreditFacilityAxis" xlink:to="us-gaap_CreditFacilityDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_TermLoanFacilityMember" xlink:label="wttr_TermLoanFacilityMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_CreditFacilityDomain" xlink:to="wttr_TermLoanFacilityMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="us-gaap_DebtInstrumentLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear" xlink:label="us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths" xlink:label="us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo" xlink:label="us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree" xlink:label="us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour" xlink:label="us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebt" xlink:label="us-gaap_LongTermDebt" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LongTermDebt" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredFinanceCostsNoncurrentNet" xlink:label="us-gaap_DeferredFinanceCostsNoncurrentNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DeferredFinanceCostsNoncurrentNet" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtCurrent" xlink:label="us-gaap_LongTermDebtCurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LongTermDebtCurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtNoncurrent" xlink:label="us-gaap_LongTermDebtNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LongTermDebtNoncurrent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd" xlink:label="us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureEquityBasedCompensationDetails" xlink:type="extended" xlink:title="99941001 - Disclosure - EQUITY-BASED COMPENSATION (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameAxis" xlink:label="us-gaap_PlanNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_PlanNameAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain" xlink:label="us-gaap_PlanNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_EquityIncentive2024PlanMember" xlink:label="wttr_EquityIncentive2024PlanMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_PlanNameDomain" xlink:to="wttr_EquityIncentive2024PlanMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_StatementClassOfStockAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember" xlink:label="us-gaap_CommonClassAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassAMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="us-gaap_SubsidiarySaleOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_SubsidiarySaleOfStockAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OverAllotmentOptionMember" xlink:label="us-gaap_OverAllotmentOptionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="us-gaap_OverAllotmentOptionMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="4" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_AwardTypeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="us-gaap_EmployeeStockOptionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_EmployeeStockOptionMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesNewIssues" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureEquityBasedCompensationEquityOptionsChangedDuringPeriodDetails" xlink:type="extended" xlink:title="99941002 - Disclosure - EQUITY-BASED COMPENSATION - Equity Options Changed During Period (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_AwardTypeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="us-gaap_EmployeeStockOptionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_EmployeeStockOptionMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOtherIncreasesDecreasesInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOtherIncreasesDecreasesInPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOtherIncreasesDecreasesInPeriod" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" xlink:to="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsOtherShareIncreaseDecreaseInPeriodWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsOtherShareIncreaseDecreaseInPeriodWeightedAverageExercisePrice" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsOtherShareIncreaseDecreaseInPeriodWeightedAverageExercisePrice" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract" xlink:label="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOtherWeightedAverageRemainingContractualTerm" xlink:label="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOtherWeightedAverageRemainingContractualTerm" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract" xlink:to="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOtherWeightedAverageRemainingContractualTerm" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingAggregateIntrinsicValueAbstract" xlink:label="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingAggregateIntrinsicValueAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingAggregateIntrinsicValueAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingAggregateIntrinsicValueAbstract" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingAggregateIntrinsicValueAbstract" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureEquityBasedCompensationEquityOptionsDetails" xlink:type="extended" xlink:title="99941003 - Disclosure - EQUITY-BASED COMPENSATION - Equity Options (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_StatementClassOfStockAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember" xlink:label="us-gaap_CommonClassAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassAMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_AwardTypeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="us-gaap_EmployeeStockOptionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_EmployeeStockOptionMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharePrice" xlink:label="us-gaap_SharePrice" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_SharePrice" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_AllocatedShareBasedCompensationExpense" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureEquityBasedCompensationRestrictedStockAwardsDetails" xlink:type="extended" xlink:title="99941004 - Disclosure - EQUITY-BASED COMPENSATION - Restricted Stock Awards (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_AwardTypeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember" xlink:label="us-gaap_CommonStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_CommonStockMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockMember" xlink:label="us-gaap_RestrictedStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_CommonStockMember" xlink:to="us-gaap_RestrictedStockMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_RestrictedStockAwardsMember" xlink:label="wttr_RestrictedStockAwardsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_RestrictedStockMember" xlink:to="wttr_RestrictedStockAwardsMember" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_AllocatedShareBasedCompensationExpense" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureEquityBasedCompensationPerformanceShareUnitsDetails" xlink:type="extended" xlink:title="99941005 - Disclosure - EQUITY-BASED COMPENSATION - Performance Share Units (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementScenarioAxis" xlink:label="srt_StatementScenarioAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="srt_StatementScenarioAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="srt_ScenarioUnspecifiedDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_StatementScenarioAxis" xlink:to="srt_ScenarioUnspecifiedDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_RelativeTotalShareholderReturnIsGreaterThanOrEqualToEightyPercentAtMaximumMember" xlink:label="wttr_RelativeTotalShareholderReturnIsGreaterThanOrEqualToEightyPercentAtMaximumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_ScenarioUnspecifiedDomain" xlink:to="wttr_RelativeTotalShareholderReturnIsGreaterThanOrEqualToEightyPercentAtMaximumMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_RelativeTotalShareholderReturnIsFiftyFivePercentAtTargetMember" xlink:label="wttr_RelativeTotalShareholderReturnIsFiftyFivePercentAtTargetMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_ScenarioUnspecifiedDomain" xlink:to="wttr_RelativeTotalShareholderReturnIsFiftyFivePercentAtTargetMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_RelativeTotalShareholderReturnIsTwentyFivePercentAtThresholdMember" xlink:label="wttr_RelativeTotalShareholderReturnIsTwentyFivePercentAtThresholdMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="srt_ScenarioUnspecifiedDomain" xlink:to="wttr_RelativeTotalShareholderReturnIsTwentyFivePercentAtThresholdMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_RelativeTotalShareholderReturnIsLessThanTwentyFivePercentAtBelowThresholdMember" xlink:label="wttr_RelativeTotalShareholderReturnIsLessThanTwentyFivePercentAtBelowThresholdMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="srt_ScenarioUnspecifiedDomain" xlink:to="wttr_RelativeTotalShareholderReturnIsLessThanTwentyFivePercentAtBelowThresholdMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_AwardTypeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember" xlink:label="us-gaap_PerformanceSharesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_PerformanceSharesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_StatementClassOfStockAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember" xlink:label="us-gaap_CommonClassAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassAMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="4" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="srt_RangeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_TargetPerformanceUnitsThatCanBeEarned" xlink:label="wttr_TargetPerformanceUnitsThatCanBeEarned" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="wttr_TargetPerformanceUnitsThatCanBeEarned" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_CommonStockReceivedInSettlementOfEachPerformanceShareUnitsTargeted" xlink:label="wttr_CommonStockReceivedInSettlementOfEachPerformanceShareUnitsTargeted" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="wttr_CommonStockReceivedInSettlementOfEachPerformanceShareUnitsTargeted" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_PercentageOfTargetAmountEarnedWhenAbsoluteTotalShareholderReturnBetweenZeroPercentAndFifteenPercent" xlink:label="wttr_PercentageOfTargetAmountEarnedWhenAbsoluteTotalShareholderReturnBetweenZeroPercentAndFifteenPercent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="wttr_PercentageOfTargetAmountEarnedWhenAbsoluteTotalShareholderReturnBetweenZeroPercentAndFifteenPercent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_PercentageOfTargetAmountEarnedWhenAbsoluteTotalShareholderReturnGreaterThanFifteenPercent" xlink:label="wttr_PercentageOfTargetAmountEarnedWhenAbsoluteTotalShareholderReturnGreaterThanFifteenPercent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="wttr_PercentageOfTargetAmountEarnedWhenAbsoluteTotalShareholderReturnGreaterThanFifteenPercent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_PercentageOfTargetAmountEarnedWhenAbsoluteTotalShareholderReturnLessThanZeroPercent" xlink:label="wttr_PercentageOfTargetAmountEarnedWhenAbsoluteTotalShareholderReturnLessThanZeroPercent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="wttr_PercentageOfTargetAmountEarnedWhenAbsoluteTotalShareholderReturnLessThanZeroPercent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceShareUnitsPeriod" xlink:label="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceShareUnitsPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceShareUnitsPeriod" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceMeasurementPeriod" xlink:label="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceMeasurementPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceMeasurementPeriod" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_AllocatedShareBasedCompensationExpense" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="12" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="13" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureEquityBasedCompensationPerformanceShareUnitsOutstandingDetails" xlink:type="extended" xlink:title="99941006 - Disclosure - EQUITY-BASED COMPENSATION - Performance share units outstanding (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_AwardTypeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember" xlink:label="us-gaap_PerformanceSharesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_PerformanceSharesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_PerformanceSharesBasedOnFreeCashFlowMember" xlink:label="wttr_PerformanceSharesBasedOnFreeCashFlowMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_PerformanceSharesMember" xlink:to="wttr_PerformanceSharesBasedOnFreeCashFlowMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_PerformanceSharesBasedOnReturnOnAssetPerformanceMember" xlink:label="wttr_PerformanceSharesBasedOnReturnOnAssetPerformanceMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_PerformanceSharesMember" xlink:to="wttr_PerformanceSharesBasedOnReturnOnAssetPerformanceMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAddedByPerformanceFactorInPeriod" xlink:label="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAddedByPerformanceFactorInPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:to="wttr_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAddedByPerformanceFactorInPeriod" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureEquityBasedCompensationShareRepurchasesDetails" xlink:type="extended" xlink:title="99941007 - Disclosure - EQUITY-BASED COMPENSATION - Share-repurchases (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_StatementClassOfStockAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember" xlink:label="us-gaap_CommonClassAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassAMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameAxis" xlink:label="us-gaap_PlanNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_PlanNameAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain" xlink:label="us-gaap_PlanNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_LongTermIncentivePlan2016Member" xlink:label="wttr_LongTermIncentivePlan2016Member" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_PlanNameDomain" xlink:to="wttr_LongTermIncentivePlan2016Member" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="srt_RangeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares" xlink:label="us-gaap_StockRepurchasedAndRetiredDuringPeriodShares" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_StockRepurchasedAndRetiredDuringPeriodShares" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation" xlink:label="us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue" xlink:label="us-gaap_StockRepurchasedAndRetiredDuringPeriodValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_StockRepurchasedAndRetiredDuringPeriodValue" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedDuringPeriodValue" xlink:label="us-gaap_StockRepurchasedDuringPeriodValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_StockRepurchasedDuringPeriodValue" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureRelatedPartyTransactionsDetails" xlink:type="extended" xlink:title="99941201 - Disclosure - RELATED-PARTY TRANSACTIONS (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionAxis" xlink:label="us-gaap_RelatedPartyTransactionAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="us-gaap_RelatedPartyTransactionAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="us-gaap_RelatedPartyTransactionDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_RelatedPartyTransactionAxis" xlink:to="us-gaap_RelatedPartyTransactionDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentMember" xlink:label="us-gaap_PropertyPlantAndEquipmentMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_RelatedPartyTransactionDomain" xlink:to="us-gaap_PropertyPlantAndEquipmentMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_RentOfCertainEquipmentOrOtherServicesMember" xlink:label="wttr_RentOfCertainEquipmentOrOtherServicesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_RelatedPartyTransactionDomain" xlink:to="wttr_RentOfCertainEquipmentOrOtherServicesMember" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_ManagementConsultingAndOtherServicesMember" xlink:label="wttr_ManagementConsultingAndOtherServicesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_RelatedPartyTransactionDomain" xlink:to="wttr_ManagementConsultingAndOtherServicesMember" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_InventoryAndConsumablesMember" xlink:label="wttr_InventoryAndConsumablesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_RelatedPartyTransactionDomain" xlink:to="wttr_InventoryAndConsumablesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_TaxReceivableAgreementMember" xlink:label="wttr_TaxReceivableAgreementMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_RelatedPartyTransactionDomain" xlink:to="wttr_TaxReceivableAgreementMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="us-gaap_RelatedPartyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="us-gaap_RelatedPartyMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_RelatedPartyMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_LegacyOwnerHoldcoAndCrestviewGPMember" xlink:label="wttr_LegacyOwnerHoldcoAndCrestviewGPMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="wttr_LegacyOwnerHoldcoAndCrestviewGPMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_ContributingLegacyOwnersMember" xlink:label="wttr_ContributingLegacyOwnersMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="wttr_ContributingLegacyOwnersMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="srt_RangeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:label="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="4" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="srt_EquityMethodInvesteeNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="srt_EquityMethodInvesteeNameDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_RelatedPartiesMember" xlink:label="wttr_RelatedPartiesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_EquityMethodInvesteeNameDomain" xlink:to="wttr_RelatedPartiesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="us-gaap_RelatedPartyTransactionLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage" xlink:label="us-gaap_EquityMethodInvestmentOwnershipPercentage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_EquityMethodInvestmentOwnershipPercentage" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Revenues" xlink:label="us-gaap_Revenues" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_Revenues" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingCostsAndExpenses" xlink:label="us-gaap_OperatingCostsAndExpenses" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_OperatingCostsAndExpenses" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_PercentageOfNetTaxSavingsForPaymentsToTaxReceivableAgreementHolders" xlink:label="wttr_PercentageOfNetTaxSavingsForPaymentsToTaxReceivableAgreementHolders" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="wttr_PercentageOfNetTaxSavingsForPaymentsToTaxReceivableAgreementHolders" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentVariableInterestRateTypeExtensibleEnumeration" xlink:label="us-gaap_DebtInstrumentVariableInterestRateTypeExtensibleEnumeration" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_DebtInstrumentVariableInterestRateTypeExtensibleEnumeration" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" xlink:label="us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_TaxReceivableAgreementsLiabilities" xlink:label="wttr_TaxReceivableAgreementsLiabilities" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="wttr_TaxReceivableAgreementsLiabilities" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureNoncontrollingInterestsEffectOfChangesDetails" xlink:type="extended" xlink:title="99941402 - Disclosure - NONCONTROLLING INTERESTS - Effect of Changes (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetTable" xlink:label="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipAxis" xlink:label="srt_OwnershipAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetTable" xlink:to="srt_OwnershipAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipDomain" xlink:label="srt_OwnershipDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_OwnershipAxis" xlink:to="srt_OwnershipDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_SesHoldingsLlcMember" xlink:label="wttr_SesHoldingsLlcMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_OwnershipDomain" xlink:to="wttr_SesHoldingsLlcMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems" xlink:label="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems" xlink:to="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangeDueToNetIncomeAttributableToParentAndEffectsOfChangesNetAbstract" xlink:label="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangeDueToNetIncomeAttributableToParentAndEffectsOfChangesNetAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestEffectsOfChangesNetLineItems" xlink:to="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangeDueToNetIncomeAttributableToParentAndEffectsOfChangesNetAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangeDueToNetIncomeAttributableToParentAndEffectsOfChangesNetAbstract" xlink:to="us-gaap_NetIncomeLoss" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangesNetAbstract" xlink:label="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangesNetAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangeDueToNetIncomeAttributableToParentAndEffectsOfChangesNetAbstract" xlink:to="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangesNetAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangesPurchaseOfInterestByParent" xlink:label="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangesPurchaseOfInterestByParent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangesNetAbstract" xlink:to="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangesPurchaseOfInterestByParent" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures" xlink:label="us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangesNetAbstract" xlink:to="us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_StockIssuedDuringPeriodValuePerformanceSharesAwardVested" xlink:label="wttr_StockIssuedDuringPeriodValuePerformanceSharesAwardVested" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangesNetAbstract" xlink:to="wttr_StockIssuedDuringPeriodValuePerformanceSharesAwardVested" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue" xlink:label="us-gaap_StockRepurchasedAndRetiredDuringPeriodValue" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangesNetAbstract" xlink:to="us-gaap_StockRepurchasedAndRetiredDuringPeriodValue" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangeDueToNetIncomeAttributableToParentAndEffectsOfChangesNet" xlink:label="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangeDueToNetIncomeAttributableToParentAndEffectsOfChangesNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangeDueToNetIncomeAttributableToParentAndEffectsOfChangesNetAbstract" xlink:to="us-gaap_ConsolidationLessThanWhollyOwnedSubsidiaryParentOwnershipInterestChangeDueToNetIncomeAttributableToParentAndEffectsOfChangesNet" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureIncomePerShareDetails" xlink:type="extended" xlink:title="99941501 - Disclosure - INCOME PER SHARE (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable" xlink:label="us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable" xlink:to="us-gaap_StatementClassOfStockAxis" priority="2" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember" xlink:label="us-gaap_CommonClassAMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassAMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember" xlink:label="us-gaap_CommonClassBMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonClassBMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable" xlink:to="us-gaap_AwardTypeAxis" priority="2" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockMember" xlink:label="us-gaap_RestrictedStockMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_RestrictedStockMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember" xlink:label="us-gaap_PerformanceSharesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_PerformanceSharesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="us-gaap_EmployeeStockOptionMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_EmployeeStockOptionMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasicLineItems" xlink:label="us-gaap_EarningsPerShareBasicLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_EarningsPerShareBasicLineItems" xlink:to="us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_EarningsPerShareBasicLineItems" xlink:to="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_PercentageOfOutstandingOptions" xlink:label="wttr_PercentageOfOutstandingOptions" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_EarningsPerShareBasicLineItems" xlink:to="wttr_PercentageOfOutstandingOptions" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfitLoss" xlink:label="us-gaap_ProfitLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_EarningsPerShareBasicLineItems" xlink:to="us-gaap_ProfitLoss" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" xlink:label="us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_EarningsPerShareBasicLineItems" xlink:to="us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_EarningsPerShareBasicLineItems" xlink:to="us-gaap_NetIncomeLoss" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_DilutiveSecuritiesEffectOnBasicEarningsPerShareRestrictiveStockAwards" xlink:label="wttr_DilutiveSecuritiesEffectOnBasicEarningsPerShareRestrictiveStockAwards" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_EarningsPerShareBasicLineItems" xlink:to="wttr_DilutiveSecuritiesEffectOnBasicEarningsPerShareRestrictiveStockAwards" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_DilutiveSecuritiesEffectOnBasicEarningsPerSharePerformanceShareUnits" xlink:label="wttr_DilutiveSecuritiesEffectOnBasicEarningsPerSharePerformanceShareUnits" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_EarningsPerShareBasicLineItems" xlink:to="wttr_DilutiveSecuritiesEffectOnBasicEarningsPerSharePerformanceShareUnits" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_DilutiveSecuritiesEffectOnBasicEarningsPerShareStockOptions" xlink:label="wttr_DilutiveSecuritiesEffectOnBasicEarningsPerShareStockOptions" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_EarningsPerShareBasicLineItems" xlink:to="wttr_DilutiveSecuritiesEffectOnBasicEarningsPerShareStockOptions" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted" xlink:label="us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_EarningsPerShareBasicLineItems" xlink:to="us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" xlink:label="us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_EarningsPerShareBasicLineItems" xlink:to="us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment" xlink:label="us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="12" xlink:from="us-gaap_EarningsPerShareBasicLineItems" xlink:to="us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:label="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="13" xlink:from="us-gaap_EarningsPerShareBasicLineItems" xlink:to="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasic" xlink:label="us-gaap_EarningsPerShareBasic" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="14" xlink:from="us-gaap_EarningsPerShareBasicLineItems" xlink:to="us-gaap_EarningsPerShareBasic" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDiluted" xlink:label="us-gaap_EarningsPerShareDiluted" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="15" xlink:from="us-gaap_EarningsPerShareBasicLineItems" xlink:to="us-gaap_EarningsPerShareDiluted" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureSegmentInformationDetails" xlink:type="extended" xlink:title="99941601 - Disclosure - SEGMENT INFORMATION (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:label="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidationItemsAxis" xlink:label="srt_ConsolidationItemsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:to="srt_ConsolidationItemsAxis" priority="2" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidationItemsDomain" xlink:label="srt_ConsolidationItemsDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_ConsolidationItemsAxis" xlink:to="srt_ConsolidationItemsDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingSegmentsMember" xlink:label="us-gaap_OperatingSegmentsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_ConsolidationItemsDomain" xlink:to="us-gaap_OperatingSegmentsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IntersegmentEliminationMember" xlink:label="us-gaap_IntersegmentEliminationMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_ConsolidationItemsDomain" xlink:to="us-gaap_IntersegmentEliminationMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" priority="2" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain" xlink:label="us-gaap_SegmentDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_TotalSegmentsMember" xlink:label="wttr_TotalSegmentsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_TotalSegmentsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_WaterServicesMember" xlink:label="wttr_WaterServicesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="wttr_TotalSegmentsMember" xlink:to="wttr_WaterServicesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_WaterInfrastructureMember" xlink:label="wttr_WaterInfrastructureMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="wttr_TotalSegmentsMember" xlink:to="wttr_WaterInfrastructureMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_ChemicalTechnologiesMember" xlink:label="wttr_ChemicalTechnologiesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="wttr_TotalSegmentsMember" xlink:to="wttr_ChemicalTechnologiesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllOtherSegmentsMember" xlink:label="us-gaap_AllOtherSegmentsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="wttr_TotalSegmentsMember" xlink:to="us-gaap_AllOtherSegmentsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="us-gaap_SegmentReportingInformationLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfReportableSegments" xlink:label="us-gaap_NumberOfReportableSegments" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_NumberOfReportableSegments" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingInformationRevenueAbstract" xlink:label="us-gaap_SegmentReportingInformationRevenueAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_SegmentReportingInformationRevenueAbstract" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SegmentReportingInformationRevenueAbstract" xlink:to="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" xlink:label="us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_SegmentReportingInformationRevenueAbstract" xlink:to="us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationAmortizationAndAccretionNet" xlink:label="us-gaap_DepreciationAmortizationAndAccretionNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_SegmentReportingInformationRevenueAbstract" xlink:to="us-gaap_DepreciationAmortizationAndAccretionNet" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SellingGeneralAndAdministrativeExpense" xlink:label="us-gaap_SellingGeneralAndAdministrativeExpense" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_SegmentReportingInformationRevenueAbstract" xlink:to="us-gaap_SellingGeneralAndAdministrativeExpense" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingOtherItemAmount" xlink:label="us-gaap_SegmentReportingOtherItemAmount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_SegmentReportingInformationRevenueAbstract" xlink:to="us-gaap_SegmentReportingOtherItemAmount" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingOtherItemCompositionDescription" xlink:label="us-gaap_SegmentReportingOtherItemCompositionDescription" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SegmentReportingOtherItemAmount" xlink:to="us-gaap_SegmentReportingOtherItemCompositionDescription" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfitLoss" xlink:label="us-gaap_ProfitLoss" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_SegmentReportingInformationRevenueAbstract" xlink:to="us-gaap_ProfitLoss" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseNonoperating" xlink:label="us-gaap_InterestExpenseNonoperating" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_SegmentReportingInformationRevenueAbstract" xlink:to="us-gaap_InterestExpenseNonoperating" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="us-gaap_IncomeTaxExpenseBenefit" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_SegmentReportingInformationRevenueAbstract" xlink:to="us-gaap_IncomeTaxExpenseBenefit" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_EarningsBeforeInterestTaxesDepreciationAndAmortization" xlink:label="wttr_EarningsBeforeInterestTaxesDepreciationAndAmortization" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_SegmentReportingInformationRevenueAbstract" xlink:to="wttr_EarningsBeforeInterestTaxesDepreciationAndAmortization" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentExpenditureAdditionToLongLivedAssets" xlink:label="us-gaap_SegmentExpenditureAdditionToLongLivedAssets" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_SegmentReportingInformationRevenueAbstract" xlink:to="us-gaap_SegmentExpenditureAdditionToLongLivedAssets" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureSegmentInformationTotalAssetsDetails" xlink:type="extended" xlink:title="99941602 - Disclosure - SEGMENT INFORMATION - Total Assets (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:label="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidationItemsAxis" xlink:label="srt_ConsolidationItemsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:to="srt_ConsolidationItemsAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidationItemsDomain" xlink:label="srt_ConsolidationItemsDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_ConsolidationItemsAxis" xlink:to="srt_ConsolidationItemsDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingSegmentsMember" xlink:label="us-gaap_OperatingSegmentsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_ConsolidationItemsDomain" xlink:to="us-gaap_OperatingSegmentsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain" xlink:label="us-gaap_SegmentDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_WaterServicesMember" xlink:label="wttr_WaterServicesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_WaterServicesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_WaterInfrastructureMember" xlink:label="wttr_WaterInfrastructureMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_WaterInfrastructureMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_ChemicalTechnologiesMember" xlink:label="wttr_ChemicalTechnologiesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_SegmentDomain" xlink:to="wttr_ChemicalTechnologiesMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllOtherSegmentsMember" xlink:label="us-gaap_AllOtherSegmentsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_SegmentDomain" xlink:to="us-gaap_AllOtherSegmentsMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="us-gaap_SegmentReportingInformationLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Assets" xlink:label="us-gaap_Assets" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_Assets" priority="1" use="optional" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://selectenergyservices.com/role/DisclosureSubsequentEventsDetails" xlink:type="extended" xlink:title="99941701 - Disclosure - SUBSEQUENT EVENTS (Details)">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTable" xlink:label="us-gaap_SubsequentEventTable" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="us-gaap_SubsequentEventTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_SubsequentEventTable" xlink:to="us-gaap_SubsequentEventTypeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="us-gaap_SubsequentEventTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_SubsequentEventTypeAxis" xlink:to="us-gaap_SubsequentEventTypeDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventMember" xlink:label="us-gaap_SubsequentEventMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SubsequentEventTypeDomain" xlink:to="us-gaap_SubsequentEventMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionAxis" xlink:label="us-gaap_AssetAcquisitionAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_SubsequentEventTable" xlink:to="us-gaap_AssetAcquisitionAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionDomain" xlink:label="us-gaap_AssetAcquisitionDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_AssetAcquisitionAxis" xlink:to="us-gaap_AssetAcquisitionDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BlackRiverRanchMember" xlink:label="wttr_BlackRiverRanchMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_AssetAcquisitionDomain" xlink:to="wttr_BlackRiverRanchMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="us-gaap_SubsequentEventTable" xlink:to="us-gaap_PropertyPlantAndEquipmentByTypeAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LandMember" xlink:label="us-gaap_LandMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_LandMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_FreeLandMember" xlink:label="wttr_FreeLandMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_LandMember" xlink:to="wttr_FreeLandMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_FederalGrazingLeaseLandMember" xlink:label="wttr_FederalGrazingLeaseLandMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_LandMember" xlink:to="wttr_FederalGrazingLeaseLandMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis" xlink:label="us-gaap_DebtInstrumentAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="4" xlink:from="us-gaap_SubsequentEventTable" xlink:to="us-gaap_DebtInstrumentAxis" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_AgriculturalLoanMember" xlink:label="wttr_AgriculturalLoanMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="wttr_AgriculturalLoanMember" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventLineItems" xlink:label="us-gaap_SubsequentEventLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_SubsequentEventLineItems" xlink:to="us-gaap_SubsequentEventTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionConsiderationTransferred" xlink:label="us-gaap_AssetAcquisitionConsiderationTransferred" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_SubsequentEventLineItems" xlink:to="us-gaap_AssetAcquisitionConsiderationTransferred" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_AssetAcquisitionsAreaOfLand" xlink:label="wttr_AssetAcquisitionsAreaOfLand" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_SubsequentEventLineItems" xlink:to="wttr_AssetAcquisitionsAreaOfLand" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_AssetAcquisitionsAreaOfWaterRights" xlink:label="wttr_AssetAcquisitionsAreaOfWaterRights" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_SubsequentEventLineItems" xlink:to="wttr_AssetAcquisitionsAreaOfWaterRights" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount" xlink:label="us-gaap_DebtInstrumentFaceAmount" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_SubsequentEventLineItems" xlink:to="us-gaap_DebtInstrumentFaceAmount" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="us-gaap_DebtInstrumentInterestRateStatedPercentage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_SubsequentEventLineItems" xlink:to="us-gaap_DebtInstrumentInterestRateStatedPercentage" priority="1" use="optional" />
    <link:loc xlink:type="locator" xlink:href="wttr-20260331.xsd#wttr_BusinessCombinationSaltWaterDisposalFacilityPermittedDisposalCapacityPerDay" xlink:label="wttr_BusinessCombinationSaltWaterDisposalFacilityPermittedDisposalCapacityPerDay" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_SubsequentEventLineItems" xlink:to="wttr_BusinessCombinationSaltWaterDisposalFacilityPermittedDisposalCapacityPerDay" priority="1" use="optional" />
  </link:definitionLink>
</link:linkbase>
