<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019edbd5-dec3-786a-998b-465021ed972d,g:c84ae459-f37a-4c2f-ac73-c9581523b205-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 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">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.coastalbank.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSUNAUDITEDParenthetical" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#CONDENSEDCONSOLIDATEDBALANCESHEETSUNAUDITEDParenthetical"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSUNAUDITEDParenthetical" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019edbd5-e61a-7c9a-89d8-eae179c0e16b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019edbd5-e61a-7008-a4fe-37898d8b8efa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61a-7c9a-89d8-eae179c0e16b" xlink:to="loc_us-gaap_StatementTable_019edbd5-e61a-7008-a4fe-37898d8b8efa" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019edbd5-e61a-757f-9191-0581975342a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019edbd5-e61a-7008-a4fe-37898d8b8efa" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019edbd5-e61a-757f-9191-0581975342a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019edbd5-e61a-757f-9191-0581975342a1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019edbd5-e61a-757f-9191-0581975342a1" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019edbd5-e61a-757f-9191-0581975342a1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019edbd5-e61a-7a7b-b84e-72ce26b5184a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019edbd5-e61a-757f-9191-0581975342a1" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019edbd5-e61a-7a7b-b84e-72ce26b5184a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubordinatedDebtMember_019edbd5-e61a-7e49-981c-e84f807ab304" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubordinatedDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019edbd5-e61a-7a7b-b84e-72ce26b5184a" xlink:to="loc_us-gaap_SubordinatedDebtMember_019edbd5-e61a-7e49-981c-e84f807ab304" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_JuniorSubordinatedDebtMember_019edbd5-e61a-79ed-8630-806ad7605600" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_JuniorSubordinatedDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019edbd5-e61a-7a7b-b84e-72ce26b5184a" xlink:to="loc_us-gaap_JuniorSubordinatedDebtMember_019edbd5-e61a-79ed-8630-806ad7605600" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019edbd5-e61a-7da0-a603-12fa0274b907" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61a-7c9a-89d8-eae179c0e16b" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019edbd5-e61a-7da0-a603-12fa0274b907" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnamortizedDebtIssuanceExpense_019edbd5-e61a-7545-b08f-d7ef979062c0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnamortizedDebtIssuanceExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61a-7c9a-89d8-eae179c0e16b" xlink:to="loc_us-gaap_UnamortizedDebtIssuanceExpense_019edbd5-e61a-7545-b08f-d7ef979062c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockNoParValue_019edbd5-e61a-7da7-a690-93eae54f59d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockNoParValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61a-7c9a-89d8-eae179c0e16b" xlink:to="loc_us-gaap_PreferredStockNoParValue_019edbd5-e61a-7da7-a690-93eae54f59d4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_019edbd5-e61a-70bb-b643-293a95c2eb78" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61a-7c9a-89d8-eae179c0e16b" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_019edbd5-e61a-70bb-b643-293a95c2eb78" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesIssued_019edbd5-e61a-7b05-a8d1-14d536c895dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61a-7c9a-89d8-eae179c0e16b" xlink:to="loc_us-gaap_PreferredStockSharesIssued_019edbd5-e61a-7b05-a8d1-14d536c895dc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_019edbd5-e61a-7194-bb7e-2d7e679d9c89" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61a-7c9a-89d8-eae179c0e16b" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_019edbd5-e61a-7194-bb7e-2d7e679d9c89" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockNoParValue_019edbd5-e61a-734c-a826-0feca4a6dd04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockNoParValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61a-7c9a-89d8-eae179c0e16b" xlink:to="loc_us-gaap_CommonStockNoParValue_019edbd5-e61a-734c-a826-0feca4a6dd04" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_019edbd5-e61a-798e-8e86-5b40141b4067" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61a-7c9a-89d8-eae179c0e16b" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_019edbd5-e61a-798e-8e86-5b40141b4067" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_019edbd5-e61a-7357-9d94-7d05504c6b6f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61a-7c9a-89d8-eae179c0e16b" xlink:to="loc_us-gaap_CommonStockSharesIssued_019edbd5-e61a-7357-9d94-7d05504c6b6f" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019edbd5-e61a-769e-a178-0d7d2c1972c0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61a-7c9a-89d8-eae179c0e16b" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019edbd5-e61a-769e-a178-0d7d2c1972c0" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFINCOMEUNAUDITED" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFINCOMEUNAUDITED"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFINCOMEUNAUDITED" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019edbd5-e61b-7565-a92c-5563f0e320e5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019edbd5-e61a-7a42-9cb4-887d98f200f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61b-7565-a92c-5563f0e320e5" xlink:to="loc_us-gaap_StatementTable_019edbd5-e61a-7a42-9cb4-887d98f200f6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019edbd5-e61a-7441-af71-58e1322537b3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019edbd5-e61a-7a42-9cb4-887d98f200f6" xlink:to="loc_srt_ProductOrServiceAxis_019edbd5-e61a-7441-af71-58e1322537b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019edbd5-e61a-7441-af71-58e1322537b3_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019edbd5-e61a-7441-af71-58e1322537b3" xlink:to="loc_srt_ProductsAndServicesDomain_019edbd5-e61a-7441-af71-58e1322537b3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019edbd5-e61a-7e1d-a62d-93ecd26ad351" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019edbd5-e61a-7441-af71-58e1322537b3" xlink:to="loc_srt_ProductsAndServicesDomain_019edbd5-e61a-7e1d-a62d-93ecd26ad351" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ServicingAndOtherBaaSFeesMember_019edbd5-e61a-75d8-8586-ac68c94d816d" xlink:href="ck1437958-20260630.xsd#ck1437958_ServicingAndOtherBaaSFeesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019edbd5-e61a-7e1d-a62d-93ecd26ad351" xlink:to="loc_ck1437958_ServicingAndOtherBaaSFeesMember_019edbd5-e61a-75d8-8586-ac68c94d816d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_TransactionFeesMember_019edbd5-e61a-7bac-a452-b79cae7e2632" xlink:href="ck1437958-20260630.xsd#ck1437958_TransactionFeesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019edbd5-e61a-7e1d-a62d-93ecd26ad351" xlink:to="loc_ck1437958_TransactionFeesMember_019edbd5-e61a-7bac-a452-b79cae7e2632" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ReimbursementOfExpensesMember_019edbd5-e61a-7430-926c-b9be1d47a90b" xlink:href="ck1437958-20260630.xsd#ck1437958_ReimbursementOfExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019edbd5-e61a-7e1d-a62d-93ecd26ad351" xlink:to="loc_ck1437958_ReimbursementOfExpensesMember_019edbd5-e61a-7430-926c-b9be1d47a90b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_BaaSCreditEnhancementsMember_019edbd5-e61a-74b2-94d5-cdd82b87f883" xlink:href="ck1437958-20260630.xsd#ck1437958_BaaSCreditEnhancementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019edbd5-e61a-7e1d-a62d-93ecd26ad351" xlink:to="loc_ck1437958_BaaSCreditEnhancementsMember_019edbd5-e61a-74b2-94d5-cdd82b87f883" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_BaaSFraudEnhancementsMember_019edbd5-e61a-7fab-ac83-e49028b344d8" xlink:href="ck1437958-20260630.xsd#ck1437958_BaaSFraudEnhancementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019edbd5-e61a-7e1d-a62d-93ecd26ad351" xlink:to="loc_ck1437958_BaaSFraudEnhancementsMember_019edbd5-e61a-7fab-ac83-e49028b344d8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_BaaSLoanExpenseMember_019edbd5-e61a-7e8c-a8bf-dec08c086fe1" xlink:href="ck1437958-20260630.xsd#ck1437958_BaaSLoanExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019edbd5-e61a-7e1d-a62d-93ecd26ad351" xlink:to="loc_ck1437958_BaaSLoanExpenseMember_019edbd5-e61a-7e8c-a8bf-dec08c086fe1" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_BaaSFraudExpenseMember_019edbd5-e61a-7b91-b21c-778e389dedc0" xlink:href="ck1437958-20260630.xsd#ck1437958_BaaSFraudExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019edbd5-e61a-7e1d-a62d-93ecd26ad351" xlink:to="loc_ck1437958_BaaSFraudExpenseMember_019edbd5-e61a-7b91-b21c-778e389dedc0" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61a-7ace-9f26-9e1408e7e85d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019edbd5-e61a-7a42-9cb4-887d98f200f6" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61a-7ace-9f26-9e1408e7e85d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e61a-7ace-9f26-9e1408e7e85d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61a-7ace-9f26-9e1408e7e85d" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e61a-7ace-9f26-9e1408e7e85d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e61a-7153-b98e-dfdaf5b41be0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61a-7ace-9f26-9e1408e7e85d" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e61a-7153-b98e-dfdaf5b41be0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_BaaSProgramFeesMember_019edbd5-e61a-7901-94a7-45c845455f99" xlink:href="ck1437958-20260630.xsd#ck1437958_BaaSProgramFeesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e61a-7153-b98e-dfdaf5b41be0" xlink:to="loc_ck1437958_BaaSProgramFeesMember_019edbd5-e61a-7901-94a7-45c845455f99" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_IndemnificationsMember_019edbd5-e61a-7f54-9adf-46d5f22fd90e" xlink:href="ck1437958-20260630.xsd#ck1437958_IndemnificationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e61a-7153-b98e-dfdaf5b41be0" xlink:to="loc_ck1437958_IndemnificationsMember_019edbd5-e61a-7f54-9adf-46d5f22fd90e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestAndDividendIncomeOperatingAbstract_019edbd5-e61b-7c1a-8c51-a74bd146e443" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestAndDividendIncomeOperatingAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61b-7565-a92c-5563f0e320e5" xlink:to="loc_us-gaap_InterestAndDividendIncomeOperatingAbstract_019edbd5-e61b-7c1a-8c51-a74bd146e443" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestAndFeeIncomeLoansAndLeases_019edbd5-e61b-735e-ac71-3bd1bfbf007a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestAndFeeIncomeLoansAndLeases"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InterestAndDividendIncomeOperatingAbstract_019edbd5-e61b-7c1a-8c51-a74bd146e443" xlink:to="loc_us-gaap_InterestAndFeeIncomeLoansAndLeases_019edbd5-e61b-735e-ac71-3bd1bfbf007a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeDepositsWithFinancialInstitutions_019edbd5-e61b-7ddf-bd22-33cf8586ae73" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestIncomeDepositsWithFinancialInstitutions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InterestAndDividendIncomeOperatingAbstract_019edbd5-e61b-7c1a-8c51-a74bd146e443" xlink:to="loc_us-gaap_InterestIncomeDepositsWithFinancialInstitutions_019edbd5-e61b-7ddf-bd22-33cf8586ae73" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeInterest_019edbd5-e61b-7ff0-a1ed-6ea5d1b20a6b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentIncomeInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InterestAndDividendIncomeOperatingAbstract_019edbd5-e61b-7c1a-8c51-a74bd146e443" xlink:to="loc_us-gaap_InvestmentIncomeInterest_019edbd5-e61b-7ff0-a1ed-6ea5d1b20a6b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_DividendsOnOtherInvestments_019edbd5-e61b-7cb4-ae24-31de1ef008a0" xlink:href="ck1437958-20260630.xsd#ck1437958_DividendsOnOtherInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InterestAndDividendIncomeOperatingAbstract_019edbd5-e61b-7c1a-8c51-a74bd146e443" xlink:to="loc_ck1437958_DividendsOnOtherInvestments_019edbd5-e61b-7cb4-ae24-31de1ef008a0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestAndDividendIncomeOperating_019edbd5-e61b-7d1a-a4bc-d0a72b8001aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestAndDividendIncomeOperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InterestAndDividendIncomeOperatingAbstract_019edbd5-e61b-7c1a-8c51-a74bd146e443" xlink:to="loc_us-gaap_InterestAndDividendIncomeOperating_019edbd5-e61b-7d1a-a4bc-d0a72b8001aa" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseAbstract_019edbd5-e61b-7655-bce7-8896a23ca19f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61b-7565-a92c-5563f0e320e5" xlink:to="loc_us-gaap_InterestExpenseAbstract_019edbd5-e61b-7655-bce7-8896a23ca19f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDeposits_019edbd5-e61b-78fb-981d-7536f7b4aba0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseDeposits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InterestExpenseAbstract_019edbd5-e61b-7655-bce7-8896a23ca19f" xlink:to="loc_us-gaap_InterestExpenseDeposits_019edbd5-e61b-78fb-981d-7536f7b4aba0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseBorrowings_019edbd5-e61b-78cd-a8ee-cd028d9ff49d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseBorrowings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InterestExpenseAbstract_019edbd5-e61b-7655-bce7-8896a23ca19f" xlink:to="loc_us-gaap_InterestExpenseBorrowings_019edbd5-e61b-78cd-a8ee-cd028d9ff49d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseOperating_019edbd5-e61b-7971-b60e-74ba0f6895eb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseOperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InterestExpenseAbstract_019edbd5-e61b-7655-bce7-8896a23ca19f" xlink:to="loc_us-gaap_InterestExpenseOperating_019edbd5-e61b-7971-b60e-74ba0f6895eb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeExpenseNet_019edbd5-e61b-7f9d-b935-0af3b77232b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestIncomeExpenseNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61b-7565-a92c-5563f0e320e5" xlink:to="loc_us-gaap_InterestIncomeExpenseNet_019edbd5-e61b-7f9d-b935-0af3b77232b7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProvisionForLoanLeaseAndOtherLosses_019edbd5-e61b-731b-af6a-4fe23de7fc47" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProvisionForLoanLeaseAndOtherLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61b-7565-a92c-5563f0e320e5" xlink:to="loc_us-gaap_ProvisionForLoanLeaseAndOtherLosses_019edbd5-e61b-731b-af6a-4fe23de7fc47" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeExpenseAfterProvisionForLoanLoss_019edbd5-e61b-7d0f-830f-77a46f14250b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestIncomeExpenseAfterProvisionForLoanLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61b-7565-a92c-5563f0e320e5" xlink:to="loc_us-gaap_InterestIncomeExpenseAfterProvisionForLoanLoss_019edbd5-e61b-7d0f-830f-77a46f14250b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoninterestIncomeAbstract_019edbd5-e61b-7ee3-82be-bf1ba793ebd5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoninterestIncomeAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61b-7565-a92c-5563f0e320e5" xlink:to="loc_us-gaap_NoninterestIncomeAbstract_019edbd5-e61b-7ee3-82be-bf1ba793ebd5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_DepositServiceChargesAndFees_019edbd5-e61b-74e7-8d6a-05db4461c3a0" xlink:href="ck1437958-20260630.xsd#ck1437958_DepositServiceChargesAndFees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestIncomeAbstract_019edbd5-e61b-7ee3-82be-bf1ba793ebd5" xlink:to="loc_ck1437958_DepositServiceChargesAndFees_019edbd5-e61b-74e7-8d6a-05db4461c3a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss_019edbd5-e61b-7e20-88ad-a0057609c723" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestIncomeAbstract_019edbd5-e61b-7ee3-82be-bf1ba793ebd5" xlink:to="loc_us-gaap_EquitySecuritiesFvNiUnrealizedGainLoss_019edbd5-e61b-7e20-88ad-a0057609c723" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoninterestIncomeOther_019edbd5-e61b-7808-9fb7-d11d5b3ef555" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoninterestIncomeOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestIncomeAbstract_019edbd5-e61b-7ee3-82be-bf1ba793ebd5" xlink:to="loc_us-gaap_NoninterestIncomeOther_019edbd5-e61b-7808-9fb7-d11d5b3ef555" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_NoninterestIncomeExcludingBaaSProgramIncomeAndBaaSIndemnificationIncome_019edbd5-e61b-735a-8b92-afca6f1cef14" xlink:href="ck1437958-20260630.xsd#ck1437958_NoninterestIncomeExcludingBaaSProgramIncomeAndBaaSIndemnificationIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestIncomeAbstract_019edbd5-e61b-7ee3-82be-bf1ba793ebd5" xlink:to="loc_ck1437958_NoninterestIncomeExcludingBaaSProgramIncomeAndBaaSIndemnificationIncome_019edbd5-e61b-735a-8b92-afca6f1cef14" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_019edbd5-e61b-7342-8ff9-c91bea030294" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestIncomeAbstract_019edbd5-e61b-7ee3-82be-bf1ba793ebd5" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_019edbd5-e61b-7342-8ff9-c91bea030294" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoninterestIncome_019edbd5-e61b-71d3-bdcb-d718565fd947" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoninterestIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestIncomeAbstract_019edbd5-e61b-7ee3-82be-bf1ba793ebd5" xlink:to="loc_us-gaap_NoninterestIncome_019edbd5-e61b-71d3-bdcb-d718565fd947" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoninterestExpenseAbstract_019edbd5-e61b-7bf8-a941-70cc5e9a9f75" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoninterestExpenseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61b-7565-a92c-5563f0e320e5" xlink:to="loc_us-gaap_NoninterestExpenseAbstract_019edbd5-e61b-7bf8-a941-70cc5e9a9f75" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LaborAndRelatedExpense_019edbd5-e61b-7397-a965-196f9a2195e9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LaborAndRelatedExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestExpenseAbstract_019edbd5-e61b-7bf8-a941-70cc5e9a9f75" xlink:to="loc_us-gaap_LaborAndRelatedExpense_019edbd5-e61b-7397-a965-196f9a2195e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OccupancyNet_019edbd5-e61b-71da-83c9-030059e119a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OccupancyNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestExpenseAbstract_019edbd5-e61b-7bf8-a941-70cc5e9a9f75" xlink:to="loc_us-gaap_OccupancyNet_019edbd5-e61b-71da-83c9-030059e119a1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_SoftwareLicensesMaintenanceAndSubscriptions_019edbd5-e61b-76f3-b06a-9c7aab2a5c29" xlink:href="ck1437958-20260630.xsd#ck1437958_SoftwareLicensesMaintenanceAndSubscriptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestExpenseAbstract_019edbd5-e61b-7bf8-a941-70cc5e9a9f75" xlink:to="loc_ck1437958_SoftwareLicensesMaintenanceAndSubscriptions_019edbd5-e61b-76f3-b06a-9c7aab2a5c29" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfessionalFees_019edbd5-e61b-77e4-a336-f89a4665528d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProfessionalFees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestExpenseAbstract_019edbd5-e61b-7bf8-a941-70cc5e9a9f75" xlink:to="loc_us-gaap_ProfessionalFees_019edbd5-e61b-77e4-a336-f89a4665528d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InformationTechnologyAndDataProcessing_019edbd5-e61b-729a-84dd-61a67654d472" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InformationTechnologyAndDataProcessing"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestExpenseAbstract_019edbd5-e61b-7bf8-a941-70cc5e9a9f75" xlink:to="loc_us-gaap_InformationTechnologyAndDataProcessing_019edbd5-e61b-729a-84dd-61a67654d472" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ExciseAndSalesTaxesCollected_019edbd5-e61b-7589-92f1-ea64bc0b32cb" xlink:href="ck1437958-20260630.xsd#ck1437958_ExciseAndSalesTaxesCollected"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestExpenseAbstract_019edbd5-e61b-7bf8-a941-70cc5e9a9f75" xlink:to="loc_ck1437958_ExciseAndSalesTaxesCollected_019edbd5-e61b-7589-92f1-ea64bc0b32cb" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_FederalDepositInsuranceCorporationPremiumExpenseBenefit_019edbd5-e61b-781d-9369-4abe4d734df2" xlink:href="ck1437958-20260630.xsd#ck1437958_FederalDepositInsuranceCorporationPremiumExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestExpenseAbstract_019edbd5-e61b-7bf8-a941-70cc5e9a9f75" xlink:to="loc_ck1437958_FederalDepositInsuranceCorporationPremiumExpenseBenefit_019edbd5-e61b-781d-9369-4abe4d734df2" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_NoninterestDirectorsAndStaffExpenses_019edbd5-e61b-7f82-8b7a-e000f7bec0c4" xlink:href="ck1437958-20260630.xsd#ck1437958_NoninterestDirectorsAndStaffExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestExpenseAbstract_019edbd5-e61b-7bf8-a941-70cc5e9a9f75" xlink:to="loc_ck1437958_NoninterestDirectorsAndStaffExpenses_019edbd5-e61b-7f82-8b7a-e000f7bec0c4" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketingExpense_019edbd5-e61b-7694-b84a-f4f46189edb8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MarketingExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestExpenseAbstract_019edbd5-e61b-7bf8-a941-70cc5e9a9f75" xlink:to="loc_us-gaap_MarketingExpense_019edbd5-e61b-7694-b84a-f4f46189edb8" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CreditEnhancementReceivableValuationAdjustment_019faafe-7315-7805-9fa3-f98ce9408acf" xlink:href="ck1437958-20260630.xsd#ck1437958_CreditEnhancementReceivableValuationAdjustment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestExpenseAbstract_019edbd5-e61b-7bf8-a941-70cc5e9a9f75" xlink:to="loc_ck1437958_CreditEnhancementReceivableValuationAdjustment_019faafe-7315-7805-9fa3-f98ce9408acf" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoninterestExpense_019edbd5-e61b-7362-863a-f957d4cc00aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherNoninterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestExpenseAbstract_019edbd5-e61b-7bf8-a941-70cc5e9a9f75" xlink:to="loc_us-gaap_OtherNoninterestExpense_019edbd5-e61b-7362-863a-f957d4cc00aa" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_NoninterestExpenseExcludingBaaSLoanAndBaaSFraudExpense_019edbd5-e61b-7c07-a03f-82c04799cfa9" xlink:href="ck1437958-20260630.xsd#ck1437958_NoninterestExpenseExcludingBaaSLoanAndBaaSFraudExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestExpenseAbstract_019edbd5-e61b-7bf8-a941-70cc5e9a9f75" xlink:to="loc_ck1437958_NoninterestExpenseExcludingBaaSLoanAndBaaSFraudExpense_019edbd5-e61b-7c07-a03f-82c04799cfa9" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_BankingServiceExpenses_019edbd5-e61b-71c7-937f-4cf3c17c068c" xlink:href="ck1437958-20260630.xsd#ck1437958_BankingServiceExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestExpenseAbstract_019edbd5-e61b-7bf8-a941-70cc5e9a9f75" xlink:to="loc_ck1437958_BankingServiceExpenses_019edbd5-e61b-71c7-937f-4cf3c17c068c" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoninterestExpense_019edbd5-e61b-72d2-88a8-629e25dfe40d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoninterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NoninterestExpenseAbstract_019edbd5-e61b-7bf8-a941-70cc5e9a9f75" xlink:to="loc_us-gaap_NoninterestExpense_019edbd5-e61b-72d2-88a8-629e25dfe40d" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019edbd5-e61b-783b-bfc7-44f60a998773" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61b-7565-a92c-5563f0e320e5" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019edbd5-e61b-783b-bfc7-44f60a998773" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019edbd5-e61b-7f8b-aa1d-dc74f652e3f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61b-7565-a92c-5563f0e320e5" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019edbd5-e61b-7f8b-aa1d-dc74f652e3f2" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019edbd5-e61b-7679-9ad7-6175c05e9891" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61b-7565-a92c-5563f0e320e5" xlink:to="loc_us-gaap_NetIncomeLoss_019edbd5-e61b-7679-9ad7-6175c05e9891" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_019edbd5-e61b-7ac1-b1c8-f31a8a992c24" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61b-7565-a92c-5563f0e320e5" xlink:to="loc_us-gaap_EarningsPerShareBasic_019edbd5-e61b-7ac1-b1c8-f31a8a992c24" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_019edbd5-e61b-76c3-bd12-fe79bd9169b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61b-7565-a92c-5563f0e320e5" xlink:to="loc_us-gaap_EarningsPerShareDiluted_019edbd5-e61b-76c3-bd12-fe79bd9169b3" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_019edbd5-e61b-70fc-90cb-6bacd96a3abe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61b-7565-a92c-5563f0e320e5" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_019edbd5-e61b-70fc-90cb-6bacd96a3abe" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019edbd5-e61b-7f2a-acb0-f50e58e3c855" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_019edbd5-e61b-70fc-90cb-6bacd96a3abe" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019edbd5-e61b-7f2a-acb0-f50e58e3c855" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019edbd5-e61b-7749-bfb9-7bcce96fcccf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_019edbd5-e61b-70fc-90cb-6bacd96a3abe" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019edbd5-e61b-7749-bfb9-7bcce96fcccf" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITYUNAUDITED" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITYUNAUDITED"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITYUNAUDITED" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019edbd5-e61b-7bb4-ada6-4fd2068d242a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019edbd5-e61b-798a-a5ac-e5cc031444ae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61b-7bb4-ada6-4fd2068d242a" xlink:to="loc_us-gaap_StatementTable_019edbd5-e61b-798a-a5ac-e5cc031444ae" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019edbd5-e61b-7c2f-867f-abd009739b29" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019edbd5-e61b-798a-a5ac-e5cc031444ae" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019edbd5-e61b-7c2f-867f-abd009739b29" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019edbd5-e61b-7c2f-867f-abd009739b29_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019edbd5-e61b-7c2f-867f-abd009739b29" xlink:to="loc_us-gaap_EquityComponentDomain_019edbd5-e61b-7c2f-867f-abd009739b29_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019edbd5-e61b-728e-aa3b-35aa16b73ecc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019edbd5-e61b-7c2f-867f-abd009739b29" xlink:to="loc_us-gaap_EquityComponentDomain_019edbd5-e61b-728e-aa3b-35aa16b73ecc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember_019edbd5-e61b-7f13-908e-cf2c00ba71ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019edbd5-e61b-728e-aa3b-35aa16b73ecc" xlink:to="loc_us-gaap_CommonStockIncludingAdditionalPaidInCapitalMember_019edbd5-e61b-7f13-908e-cf2c00ba71ed" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019edbd5-e61b-7a7c-af74-0c7c22730616" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019edbd5-e61b-728e-aa3b-35aa16b73ecc" xlink:to="loc_us-gaap_RetainedEarningsMember_019edbd5-e61b-7a7c-af74-0c7c22730616" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019edbd5-e61b-7cb2-9846-7410d295d3a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019edbd5-e61b-728e-aa3b-35aa16b73ecc" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019edbd5-e61b-7cb2-9846-7410d295d3a1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019edbd5-e61b-7ddf-9b0c-85bd513511c1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019edbd5-e61b-7bb4-ada6-4fd2068d242a" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019edbd5-e61b-7ddf-9b0c-85bd513511c1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_019edbd5-e61b-7e17-82e5-8cff7f199c30" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019edbd5-e61b-7ddf-9b0c-85bd513511c1" xlink:to="loc_us-gaap_SharesOutstanding_019edbd5-e61b-7e17-82e5-8cff7f199c30" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019edbd5-e61b-7be2-b859-e005bbaf406c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019edbd5-e61b-7ddf-9b0c-85bd513511c1" xlink:to="loc_us-gaap_StockholdersEquity_019edbd5-e61b-7be2-b859-e005bbaf406c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019edbd5-e61b-723b-8ee7-7f2038403e37" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019edbd5-e61b-7ddf-9b0c-85bd513511c1" xlink:to="loc_us-gaap_NetIncomeLoss_019edbd5-e61b-723b-8ee7-7f2038403e37" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross_019faaec-11b2-7a2b-a066-4fb5509ff0c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019edbd5-e61b-7ddf-9b0c-85bd513511c1" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross_019faaec-11b2-7a2b-a066-4fb5509ff0c4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_StockIssuedDuringPeriodSharesVestingOfRestrictedStockUnits_019edbd5-e61b-77d1-8b05-036ef5613c8f" xlink:href="ck1437958-20260630.xsd#ck1437958_StockIssuedDuringPeriodSharesVestingOfRestrictedStockUnits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019edbd5-e61b-7ddf-9b0c-85bd513511c1" xlink:to="loc_ck1437958_StockIssuedDuringPeriodSharesVestingOfRestrictedStockUnits_019edbd5-e61b-77d1-8b05-036ef5613c8f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_StockIssuedDuringPeriodValueVestingOfRestrictedStockUnits_019edbd5-e61b-7daa-87cd-91f4f09adba9" xlink:href="ck1437958-20260630.xsd#ck1437958_StockIssuedDuringPeriodValueVestingOfRestrictedStockUnits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019edbd5-e61b-7ddf-9b0c-85bd513511c1" xlink:to="loc_ck1437958_StockIssuedDuringPeriodValueVestingOfRestrictedStockUnits_019edbd5-e61b-7daa-87cd-91f4f09adba9" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019edbd5-e61b-7150-b536-5a80037f4d4c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019edbd5-e61b-7ddf-9b0c-85bd513511c1" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019edbd5-e61b-7150-b536-5a80037f4d4c" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019edbd5-e61b-76c7-b627-5cf912209648" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019edbd5-e61b-7ddf-9b0c-85bd513511c1" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019edbd5-e61b-76c7-b627-5cf912209648" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019edbd5-e61b-711b-98b8-337f895c754f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019edbd5-e61b-7ddf-9b0c-85bd513511c1" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019edbd5-e61b-711b-98b8-337f895c754f" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_019edbd5-e61b-7dd3-8754-7cc27b06e079" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019edbd5-e61b-7ddf-9b0c-85bd513511c1" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_019edbd5-e61b-7dd3-8754-7cc27b06e079" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_StockIssuedDuringPeriodValueStockOptionsExercisedNet_019faaf0-2a2a-7971-b4f4-ac4496c04e8c" xlink:href="ck1437958-20260630.xsd#ck1437958_StockIssuedDuringPeriodValueStockOptionsExercisedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019edbd5-e61b-7ddf-9b0c-85bd513511c1" xlink:to="loc_ck1437958_StockIssuedDuringPeriodValueStockOptionsExercisedNet_019faaf0-2a2a-7971-b4f4-ac4496c04e8c" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019edbd5-e61b-7334-a044-edc1b451ba7b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019edbd5-e61b-7ddf-9b0c-85bd513511c1" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019edbd5-e61b-7334-a044-edc1b451ba7b" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_019edbd5-e61b-797e-a6e3-427d7b13efb1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019edbd5-e61b-7cbf-8c38-8313b7e4d4a0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITYUNAUDITEDPARENTHETICAL" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITYUNAUDITEDPARENTHETICAL"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSHAREHOLDERSEQUITYUNAUDITEDPARENTHETICAL" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019faaea-8ddd-7d02-a9f6-39b9ed6fbb88" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019faaea-8ddc-7dfb-937f-d2881156b577" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019faaea-8ddd-7d02-a9f6-39b9ed6fbb88" xlink:to="loc_us-gaap_StatementTable_019faaea-8ddc-7dfb-937f-d2881156b577" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019faaea-8ddc-750f-8b38-fdb6b924e8ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019faaea-8ddc-7dfb-937f-d2881156b577" xlink:to="loc_us-gaap_AwardTypeAxis_019faaea-8ddc-750f-8b38-fdb6b924e8ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019faaea-8ddc-750f-8b38-fdb6b924e8ff_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019faaea-8ddc-750f-8b38-fdb6b924e8ff" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019faaea-8ddc-750f-8b38-fdb6b924e8ff_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019faaea-8ddc-7d10-a303-000693ad3ac4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019faaea-8ddc-750f-8b38-fdb6b924e8ff" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019faaea-8ddc-7d10-a303-000693ad3ac4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_RestrictedStockAwardsMember_019fcd8d-6094-7822-885b-855fd7a47258" xlink:href="ck1437958-20260630.xsd#ck1437958_RestrictedStockAwardsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019faaea-8ddc-7d10-a303-000693ad3ac4" xlink:to="loc_ck1437958_RestrictedStockAwardsMember_019fcd8d-6094-7822-885b-855fd7a47258" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_019faaea-8ddc-7074-987f-6f9162fb6627" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019faaea-8ddc-7d10-a303-000693ad3ac4" xlink:to="loc_us-gaap_RestrictedStockMember_019faaea-8ddc-7074-987f-6f9162fb6627" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019faaeb-074b-7e02-8285-b9c0bdc14cab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019faaea-8ddc-7d10-a303-000693ad3ac4" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019faaeb-074b-7e02-8285-b9c0bdc14cab" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_019faaea-8ddc-7609-98be-a5bfe9157bb2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019faaea-8ddc-7dfb-937f-d2881156b577" xlink:to="loc_us-gaap_VestingAxis_019faaea-8ddc-7609-98be-a5bfe9157bb2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_019faaea-8ddc-7609-98be-a5bfe9157bb2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_019faaea-8ddc-7609-98be-a5bfe9157bb2" xlink:to="loc_us-gaap_VestingDomain_019faaea-8ddc-7609-98be-a5bfe9157bb2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_019faaea-8ddc-7363-b878-7817f0535b14" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_019faaea-8ddc-7609-98be-a5bfe9157bb2" xlink:to="loc_us-gaap_VestingDomain_019faaea-8ddc-7363-b878-7817f0535b14" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_019faaea-8ddd-766f-b427-76bd0ef790ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationAwardTrancheTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_019faaea-8ddc-7363-b878-7817f0535b14" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_019faaea-8ddd-766f-b427-76bd0ef790ac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_019faaea-8ddd-74bb-8d52-b659aace1f86" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019faaea-8ddd-7d02-a9f6-39b9ed6fbb88" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_019faaea-8ddd-74bb-8d52-b659aace1f86" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/InvestmentsSecuritiesAmortizedCostandFairValuesofInvestmentsinDebtSecuritiesDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#InvestmentsSecuritiesAmortizedCostandFairValuesofInvestmentsinDebtSecuritiesDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/InvestmentsSecuritiesAmortizedCostandFairValuesofInvestmentsinDebtSecuritiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019edbd5-e61b-7f3c-a87c-2482ac71c9e5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019edbd5-e61b-79d7-a947-29f2b9e3ee73" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019edbd5-e61b-7f3c-a87c-2482ac71c9e5" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019edbd5-e61b-79d7-a947-29f2b9e3ee73" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-7d49-b74b-764bdbe9f650" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019edbd5-e61b-79d7-a947-29f2b9e3ee73" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-7d49-b74b-764bdbe9f650" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-7d49-b74b-764bdbe9f650_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-7d49-b74b-764bdbe9f650" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-7d49-b74b-764bdbe9f650_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-75f9-9d9f-439844c78f73" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-7d49-b74b-764bdbe9f650" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-75f9-9d9f-439844c78f73" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019edbd5-e61b-7826-84f9-4cc52cedfdb6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-75f9-9d9f-439844c78f73" xlink:to="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019edbd5-e61b-7826-84f9-4cc52cedfdb6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialMortgageBackedSecuritiesMember_019edbd5-e61b-78d1-bc97-e2a81800592f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResidentialMortgageBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-75f9-9d9f-439844c78f73" xlink:to="loc_us-gaap_ResidentialMortgageBackedSecuritiesMember_019edbd5-e61b-78d1-bc97-e2a81800592f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-7d5b-978d-03f20fa64fe1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019edbd5-e61b-7f3c-a87c-2482ac71c9e5" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-7d5b-978d-03f20fa64fe1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019edbd5-e61b-7541-afcf-5121b0ddb4b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019edbd5-e61b-7f3c-a87c-2482ac71c9e5" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019edbd5-e61b-7541-afcf-5121b0ddb4b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019edbd5-e61b-73d1-9a88-8785baa3ea75" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019edbd5-e61b-7f3c-a87c-2482ac71c9e5" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019edbd5-e61b-73d1-9a88-8785baa3ea75" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_019edbd5-e61b-71b0-a1a3-1abb13fe1290" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019edbd5-e61b-7f3c-a87c-2482ac71c9e5" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_019edbd5-e61b-71b0-a1a3-1abb13fe1290" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostAllowanceForCreditLossExcludingAccruedInterest_019edbd5-e61b-73e1-9ccb-5ec3b3d964ab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostAllowanceForCreditLossExcludingAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019edbd5-e61b-7f3c-a87c-2482ac71c9e5" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostAllowanceForCreditLossExcludingAccruedInterest_019edbd5-e61b-73e1-9ccb-5ec3b3d964ab" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_019edbd5-e61b-7440-90df-fbb56c18bdee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesTable_019edbd5-e61b-7ea9-bbed-d9d0ab56d35c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfHeldToMaturitySecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_019edbd5-e61b-7440-90df-fbb56c18bdee" xlink:to="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesTable_019edbd5-e61b-7ea9-bbed-d9d0ab56d35c" xlink:type="arc" order="2" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-7fbd-a7ef-b0213b1e9243" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesTable_019edbd5-e61b-7ea9-bbed-d9d0ab56d35c" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-7fbd-a7ef-b0213b1e9243" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-7fbd-a7ef-b0213b1e9243_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-765b-9fe3-55d977b4b1f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialMortgageBackedSecuritiesMember_019edbd5-e61b-7796-9c6f-920c75f21712" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResidentialMortgageBackedSecuritiesMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesAbstract_019edbd5-e61b-77de-8109-badb39fc2319" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_019edbd5-e61b-7440-90df-fbb56c18bdee" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesAbstract_019edbd5-e61b-77de-8109-badb39fc2319" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-7048-838a-af5ff8c622c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestBeforeAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesAbstract_019edbd5-e61b-77de-8109-badb39fc2319" xlink:to="loc_us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-7048-838a-af5ff8c622c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain_019edbd5-e61b-7dca-9ef1-d5bfe88b1fbe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesAbstract_019edbd5-e61b-77de-8109-badb39fc2319" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain_019edbd5-e61b-7dca-9ef1-d5bfe88b1fbe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss_019edbd5-e61b-7cbc-8bd0-9fe36220721b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesAbstract_019edbd5-e61b-77de-8109-badb39fc2319" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss_019edbd5-e61b-7cbc-8bd0-9fe36220721b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesFairValue_019edbd5-e61b-77d8-9587-e41104148dc9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesAbstract_019edbd5-e61b-77de-8109-badb39fc2319" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesFairValue_019edbd5-e61b-77d8-9587-e41104148dc9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLossExcludingAccruedInterest_019edbd5-e61b-7af6-9eed-13163e6d4bdf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLossExcludingAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesAbstract_019edbd5-e61b-77de-8109-badb39fc2319" xlink:to="loc_us-gaap_DebtSecuritiesHeldToMaturityAllowanceForCreditLossExcludingAccruedInterest_019edbd5-e61b-7af6-9eed-13163e6d4bdf" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAndHeldToMaturityAmortizedCostBeforeAllowanceForCreditLoss_019edbd5-e61b-7f4e-bdd5-5d3696936681" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAndHeldToMaturityAmortizedCostBeforeAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_019edbd5-e61b-7440-90df-fbb56c18bdee" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAndHeldToMaturityAmortizedCostBeforeAllowanceForCreditLoss_019edbd5-e61b-7f4e-bdd5-5d3696936681" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_DebtSecuritiesAvailableForSaleAndHeldToMaturityAccumulatedUnrecognizedGain_019edbd5-e61b-76ec-92b1-95a037d78047" xlink:href="ck1437958-20260630.xsd#ck1437958_DebtSecuritiesAvailableForSaleAndHeldToMaturityAccumulatedUnrecognizedGain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_019edbd5-e61b-7440-90df-fbb56c18bdee" xlink:to="loc_ck1437958_DebtSecuritiesAvailableForSaleAndHeldToMaturityAccumulatedUnrecognizedGain_019edbd5-e61b-76ec-92b1-95a037d78047" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_DebtSecuritiesAvailableForSaleAndHeldToMaturityAccumulatedGrossUnrealizedLossBeforeTax_019edbd5-e61b-79cb-b75d-b9ae30d63af6" xlink:href="ck1437958-20260630.xsd#ck1437958_DebtSecuritiesAvailableForSaleAndHeldToMaturityAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_019edbd5-e61b-7440-90df-fbb56c18bdee" xlink:to="loc_ck1437958_DebtSecuritiesAvailableForSaleAndHeldToMaturityAccumulatedGrossUnrealizedLossBeforeTax_019edbd5-e61b-79cb-b75d-b9ae30d63af6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAndHeldToMaturityFairValue_019edbd5-e61b-7520-a27b-69fe4841cd93" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAndHeldToMaturityFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_019edbd5-e61b-7440-90df-fbb56c18bdee" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAndHeldToMaturityFairValue_019edbd5-e61b-7520-a27b-69fe4841cd93" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAndHeldToMaturityAllowanceForCreditLoss_019edbd5-e61b-7906-bccb-a4eee61207ef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAndHeldToMaturityAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_019edbd5-e61b-7440-90df-fbb56c18bdee" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAndHeldToMaturityAllowanceForCreditLoss_019edbd5-e61b-7906-bccb-a4eee61207ef" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/InvestmentsSecuritiesAmortizedCostandFairValueofDebtSecuritiesbyContractualMaturityDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#InvestmentsSecuritiesAmortizedCostandFairValueofDebtSecuritiesbyContractualMaturityDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/InvestmentsSecuritiesAmortizedCostandFairValueofDebtSecuritiesbyContractualMaturityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019edbd5-e61b-71d2-b070-d3419060c18e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019edbd5-e61b-7378-a4bb-bc14f4d73f54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019edbd5-e61b-71d2-b070-d3419060c18e" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019edbd5-e61b-7378-a4bb-bc14f4d73f54" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-7425-a0a9-5c204b3ca794" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019edbd5-e61b-7378-a4bb-bc14f4d73f54" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-7425-a0a9-5c204b3ca794" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-7425-a0a9-5c204b3ca794_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-7425-a0a9-5c204b3ca794" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-7425-a0a9-5c204b3ca794_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-7f60-bcf4-c778cc75715b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-7425-a0a9-5c204b3ca794" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-7f60-bcf4-c778cc75715b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_USAgencyResidentialMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember_019edbd5-e61b-7ee9-a485-57fe55bbc75b" xlink:href="ck1437958-20260630.xsd#ck1437958_USAgencyResidentialMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-7f60-bcf4-c778cc75715b" xlink:to="loc_ck1437958_USAgencyResidentialMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember_019edbd5-e61b-7ee9-a485-57fe55bbc75b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAmortizedCostAbstract_019edbd5-e61b-75c8-89ef-e46c059d97a7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAmortizedCostAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019edbd5-e61b-71d2-b070-d3419060c18e" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAmortizedCostAbstract_019edbd5-e61b-75c8-89ef-e46c059d97a7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost_019edbd5-e61b-7d39-b6ac-82187a01c3e9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAmortizedCostAbstract_019edbd5-e61b-75c8-89ef-e46c059d97a7" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost_019edbd5-e61b-7d39-b6ac-82187a01c3e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-708a-9bda-2e71ae817356" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAmortizedCostAbstract_019edbd5-e61b-75c8-89ef-e46c059d97a7" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-708a-9bda-2e71ae817356" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_019edbd5-e61b-7381-87e4-3642f62e027b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019edbd5-e61b-71d2-b070-d3419060c18e" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_019edbd5-e61b-7381-87e4-3642f62e027b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue_019edbd5-e61b-715e-9f9d-4e28c542621b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_019edbd5-e61b-7381-87e4-3642f62e027b" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue_019edbd5-e61b-715e-9f9d-4e28c542621b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_019edbd5-e61b-77a6-9fbe-8bc05b913fb9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_019edbd5-e61b-7381-87e4-3642f62e027b" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_019edbd5-e61b-77a6-9fbe-8bc05b913fb9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_019edbd5-e61b-745c-b33f-bb767221cec2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesTable_019edbd5-e61b-712f-9ebd-dece0f0d47d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfHeldToMaturitySecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_019edbd5-e61b-745c-b33f-bb767221cec2" xlink:to="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesTable_019edbd5-e61b-712f-9ebd-dece0f0d47d2" xlink:type="arc" order="2" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-71ef-9c15-03bcd476bb9a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesTable_019edbd5-e61b-712f-9ebd-dece0f0d47d2" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-71ef-9c15-03bcd476bb9a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-71ef-9c15-03bcd476bb9a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-705e-8fa3-e8798b427960" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_USAgencyResidentialMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember_019edbd5-e61b-79e2-8bd1-6a69cb9d5367" xlink:href="ck1437958-20260630.xsd#ck1437958_USAgencyResidentialMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateAbstract_019edbd5-e61b-768a-b3c0-467c50f04530" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_019edbd5-e61b-745c-b33f-bb767221cec2" xlink:to="loc_us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateAbstract_019edbd5-e61b-768a-b3c0-467c50f04530" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityWithoutSingleMaturityDate_019edbd5-e61b-77b4-9d62-a944656b1a97" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityWithoutSingleMaturityDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateAbstract_019edbd5-e61b-768a-b3c0-467c50f04530" xlink:to="loc_us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityWithoutSingleMaturityDate_019edbd5-e61b-77b4-9d62-a944656b1a97" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-7809-b3c5-921633cf7f26" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestBeforeAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesHeldToMaturityAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossMaturityAllocatedAndSingleMaturityDateAbstract_019edbd5-e61b-768a-b3c0-467c50f04530" xlink:to="loc_us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-7809-b3c5-921633cf7f26" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesFairValueAbstract_019edbd5-e61b-7c28-8c22-e573f8e66384" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesFairValueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_019edbd5-e61b-745c-b33f-bb767221cec2" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesFairValueAbstract_019edbd5-e61b-7c28-8c22-e573f8e66384" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue_019edbd5-e61b-71b6-84a4-febeaa68c801" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesFairValueAbstract_019edbd5-e61b-7c28-8c22-e573f8e66384" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue_019edbd5-e61b-71b6-84a4-febeaa68c801" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesFairValue_019edbd5-e61b-7999-bc9f-bca66c238b8f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesDebtMaturitiesFairValueAbstract_019edbd5-e61b-7c28-8c22-e573f8e66384" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesFairValue_019edbd5-e61b-7999-bc9f-bca66c238b8f" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/InvestmentsSecuritiesSummaryofInvestmentSecuritiesContinuousUnrealizedLossPositionDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#InvestmentsSecuritiesSummaryofInvestmentSecuritiesContinuousUnrealizedLossPositionDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/InvestmentsSecuritiesSummaryofInvestmentSecuritiesContinuousUnrealizedLossPositionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019edbd5-e61b-7dfb-bc5c-fbdbd4d30e48" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019edbd5-e61b-7028-8c36-3be18fe1cc6b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019edbd5-e61b-7dfb-bc5c-fbdbd4d30e48" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019edbd5-e61b-7028-8c36-3be18fe1cc6b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-7dc9-8175-b383a62b87a3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019edbd5-e61b-7028-8c36-3be18fe1cc6b" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-7dc9-8175-b383a62b87a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-7dc9-8175-b383a62b87a3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-7dc9-8175-b383a62b87a3" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-7dc9-8175-b383a62b87a3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-7e0f-a67e-c6948ddcd352" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-7dc9-8175-b383a62b87a3" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-7e0f-a67e-c6948ddcd352" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019edbd5-e61b-7250-90b1-b8cbce489a32" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-7e0f-a67e-c6948ddcd352" xlink:to="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019edbd5-e61b-7250-90b1-b8cbce489a32" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialMortgageBackedSecuritiesMember_019edbd5-e61b-7ef7-9ec3-c195b25ce6ae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResidentialMortgageBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-7e0f-a67e-c6948ddcd352" xlink:to="loc_us-gaap_ResidentialMortgageBackedSecuritiesMember_019edbd5-e61b-7ef7-9ec3-c195b25ce6ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019edbd5-e61b-701e-9778-5d6c04df69c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019edbd5-e61b-7dfb-bc5c-fbdbd4d30e48" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019edbd5-e61b-701e-9778-5d6c04df69c4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_019edbd5-e61b-79bb-a61c-2abccec562f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019edbd5-e61b-701e-9778-5d6c04df69c4" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_019edbd5-e61b-79bb-a61c-2abccec562f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019edbd5-e61b-76d4-b703-488af359423b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019edbd5-e61b-701e-9778-5d6c04df69c4" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019edbd5-e61b-76d4-b703-488af359423b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_019edbd5-e61b-7ee0-9bfd-7a85bc8576db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019edbd5-e61b-701e-9778-5d6c04df69c4" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_019edbd5-e61b-7ee0-9bfd-7a85bc8576db" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019edbd5-e61b-7422-8be7-f0b514284cb4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019edbd5-e61b-701e-9778-5d6c04df69c4" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019edbd5-e61b-7422-8be7-f0b514284cb4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_019edbd5-e61b-7f01-a164-a08a561a82be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019edbd5-e61b-701e-9778-5d6c04df69c4" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_019edbd5-e61b-7f01-a164-a08a561a82be" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019edbd5-e61b-7fd9-835c-55ef0514b321" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019edbd5-e61b-701e-9778-5d6c04df69c4" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019edbd5-e61b-7fd9-835c-55ef0514b321" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_019edbd5-e61b-73a2-9e87-19d99c2d9a61" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesTable_019edbd5-e61b-7a4f-80b5-96a0b3848f3a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfHeldToMaturitySecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_019edbd5-e61b-73a2-9e87-19d99c2d9a61" xlink:to="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesTable_019edbd5-e61b-7a4f-80b5-96a0b3848f3a" xlink:type="arc" order="2" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-7453-b78f-b3d48c3e1b11" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesTable_019edbd5-e61b-7a4f-80b5-96a0b3848f3a" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019edbd5-e61b-7453-b78f-b3d48c3e1b11" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-7453-b78f-b3d48c3e1b11_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019edbd5-e61b-75da-88ef-1326747d3459" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialMortgageBackedSecuritiesMember_019edbd5-e61b-7b3f-a63f-c086f70a7ad8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResidentialMortgageBackedSecuritiesMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019edbd5-e61b-7891-856c-07fe75f584b4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfHeldToMaturitySecuritiesLineItems_019edbd5-e61b-73a2-9e87-19d99c2d9a61" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019edbd5-e61b-7891-856c-07fe75f584b4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue_019edbd5-e61b-7cb4-97a3-84d48e681ea9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019edbd5-e61b-7891-856c-07fe75f584b4" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue_019edbd5-e61b-7cb4-97a3-84d48e681ea9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019edbd5-e61b-73ec-a49e-b097cb7d1949" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019edbd5-e61b-7891-856c-07fe75f584b4" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019edbd5-e61b-73ec-a49e-b097cb7d1949" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue_019edbd5-e61b-7029-bcb5-234325147e8f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019edbd5-e61b-7891-856c-07fe75f584b4" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue_019edbd5-e61b-7029-bcb5-234325147e8f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019edbd5-e61b-7c41-abb5-12d86419dbcf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019edbd5-e61b-7891-856c-07fe75f584b4" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019edbd5-e61b-7c41-abb5-12d86419dbcf" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue_019edbd5-e61b-7bdd-8318-36eb447cdb1c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019edbd5-e61b-7891-856c-07fe75f584b4" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue_019edbd5-e61b-7bdd-8318-36eb447cdb1c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss_019edbd5-e61b-70a1-ab61-17cd7da9dc75" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019edbd5-e61b-7891-856c-07fe75f584b4" xlink:to="loc_us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss_019edbd5-e61b-70a1-ab61-17cd7da9dc75" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesAdditionalInformationDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#LoansandAllowanceforCreditLossesAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-72d8-9e0b-79e43315f285" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-72d8-9e0b-79e43315f285" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019edbd5-e61b-7a91-bf2b-21b3b3602365" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-72d8-9e0b-79e43315f285" xlink:to="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019edbd5-e61b-7a91-bf2b-21b3b3602365" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-7a91-bf2b-21b3b3602365_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019edbd5-e61b-7a91-bf2b-21b3b3602365" xlink:to="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-7a91-bf2b-21b3b3602365_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-7916-9e3b-35194e5c15d6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019edbd5-e61b-7a91-bf2b-21b3b3602365" xlink:to="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-7916-9e3b-35194e5c15d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXLoansMember_019edbd5-e61b-7e49-bd25-c073da963fd7" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-7916-9e3b-35194e5c15d6" xlink:to="loc_ck1437958_CCBXLoansMember_019edbd5-e61b-7e49-bd25-c073da963fd7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditCardMember_019edbd5-e61b-7796-8cda-ac061f9875ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditCardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-7916-9e3b-35194e5c15d6" xlink:to="loc_us-gaap_CreditCardMember_019edbd5-e61b-7796-8cda-ac061f9875ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXCapitalCallLinesMember_019edbd5-e61b-79de-ab4d-aa6d3dc7bac7" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXCapitalCallLinesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-7916-9e3b-35194e5c15d6" xlink:to="loc_ck1437958_CCBXCapitalCallLinesMember_019edbd5-e61b-79de-ab4d-aa6d3dc7bac7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_UnsecuredCCBXLoansMember_019edbd5-e61b-76b2-b064-50e376e7ae35" xlink:href="ck1437958-20260630.xsd#ck1437958_UnsecuredCCBXLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-7916-9e3b-35194e5c15d6" xlink:to="loc_ck1437958_UnsecuredCCBXLoansMember_019edbd5-e61b-76b2-b064-50e376e7ae35" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7342-ae6e-3164272ca948" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-72d8-9e0b-79e43315f285" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7342-ae6e-3164272ca948" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7342-ae6e-3164272ca948_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7342-ae6e-3164272ca948" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7342-ae6e-3164272ca948_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-73ac-b91f-a517205a8958" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7342-ae6e-3164272ca948" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-73ac-b91f-a517205a8958" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019edbd5-e61b-7461-81a4-77eb93520474" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-73ac-b91f-a517205a8958" xlink:to="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019edbd5-e61b-7461-81a4-77eb93520474" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-728d-8a02-b024768ede65" xlink:href="ck1437958-20260630.xsd#ck1437958_ConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-73ac-b91f-a517205a8958" xlink:to="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-728d-8a02-b024768ede65" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommunityBankMember_019edbd5-e61b-7b48-9a92-ca3287841ba4" xlink:href="ck1437958-20260630.xsd#ck1437958_CommunityBankMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-73ac-b91f-a517205a8958" xlink:to="loc_ck1437958_CommunityBankMember_019edbd5-e61b-7b48-9a92-ca3287841ba4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXMember_019edbd5-e61b-72c9-8a81-08aeed68cf97" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-73ac-b91f-a517205a8958" xlink:to="loc_ck1437958_CCBXMember_019edbd5-e61b-72c9-8a81-08aeed68cf97" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsumerLoanMember_019edbd5-e61b-7ef9-a8a0-51d9bcb46444" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConsumerLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-73ac-b91f-a517205a8958" xlink:to="loc_us-gaap_ConsumerLoanMember_019edbd5-e61b-7ef9-a8a0-51d9bcb46444" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsumerPortfolioSegmentMember_019edbd5-e61b-719b-9949-6d513f0a3c52" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConsumerPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-73ac-b91f-a517205a8958" xlink:to="loc_us-gaap_ConsumerPortfolioSegmentMember_019edbd5-e61b-719b-9949-6d513f0a3c52" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPortfolioSegmentMember_019edbd5-e61b-7023-ad4b-d104da32fb6d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-73ac-b91f-a517205a8958" xlink:to="loc_us-gaap_CommercialPortfolioSegmentMember_019edbd5-e61b-7023-ad4b-d104da32fb6d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember_019edbd5-e61b-7928-9bcb-76ed9efaa9fb" xlink:href="ck1437958-20260630.xsd#ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-73ac-b91f-a517205a8958" xlink:to="loc_ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember_019edbd5-e61b-7928-9bcb-76ed9efaa9fb" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-7def-ae43-881e6c1e568e" xlink:href="ck1437958-20260630.xsd#ck1437958_RealEstatePortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-73ac-b91f-a517205a8958" xlink:to="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-7def-ae43-881e6c1e568e" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PledgedStatusAxis_019edbd5-e61b-7713-916e-054d6c599c5d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PledgedStatusAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-72d8-9e0b-79e43315f285" xlink:to="loc_us-gaap_PledgedStatusAxis_019edbd5-e61b-7713-916e-054d6c599c5d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PledgedStatusDomain_019edbd5-e61b-7713-916e-054d6c599c5d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PledgedStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PledgedStatusAxis_019edbd5-e61b-7713-916e-054d6c599c5d" xlink:to="loc_us-gaap_PledgedStatusDomain_019edbd5-e61b-7713-916e-054d6c599c5d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PledgedStatusDomain_019edbd5-e61b-7141-a138-738a305c1bd5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PledgedStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PledgedStatusAxis_019edbd5-e61b-7713-916e-054d6c599c5d" xlink:to="loc_us-gaap_PledgedStatusDomain_019edbd5-e61b-7141-a138-738a305c1bd5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetPledgedAsCollateralMember_019edbd5-e61b-72bf-8de3-c130be21e843" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetPledgedAsCollateralMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PledgedStatusDomain_019edbd5-e61b-7141-a138-738a305c1bd5" xlink:to="loc_us-gaap_AssetPledgedAsCollateralMember_019edbd5-e61b-72bf-8de3-c130be21e843" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7f0d-99c0-ce7647ecc2a6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-72d8-9e0b-79e43315f285" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7f0d-99c0-ce7647ecc2a6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7f0d-99c0-ce7647ecc2a6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7f0d-99c0-ce7647ecc2a6" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7f0d-99c0-ce7647ecc2a6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-769f-be97-350b6f4326fb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7f0d-99c0-ce7647ecc2a6" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-769f-be97-350b6f4326fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_InstallmentClosedEndConsumerLoansMember_019edbd5-e61b-7402-9445-023544c592d3" xlink:href="ck1437958-20260630.xsd#ck1437958_InstallmentClosedEndConsumerLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-769f-be97-350b6f4326fb" xlink:to="loc_ck1437958_InstallmentClosedEndConsumerLoansMember_019edbd5-e61b-7402-9445-023544c592d3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_RevolvingOpenEndConsumerLoansMember_019edbd5-e61b-74cc-9321-0705d9a88bde" xlink:href="ck1437958-20260630.xsd#ck1437958_RevolvingOpenEndConsumerLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-769f-be97-350b6f4326fb" xlink:to="loc_ck1437958_RevolvingOpenEndConsumerLoansMember_019edbd5-e61b-74cc-9321-0705d9a88bde" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnfundedLoanCommitmentMember_019edbd5-e61b-7d40-bde2-e18bf95fd413" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnfundedLoanCommitmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-769f-be97-350b6f4326fb" xlink:to="loc_us-gaap_UnfundedLoanCommitmentMember_019edbd5-e61b-7d40-bde2-e18bf95fd413" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialRealEstateLoansMember_019edbd5-e61b-76a0-a4c6-6c0380b434c8" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-769f-be97-350b6f4326fb" xlink:to="loc_ck1437958_CommercialRealEstateLoansMember_019edbd5-e61b-76a0-a4c6-6c0380b434c8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableMember_019edbd5-e61b-7523-8adf-f64f7d63be9b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-769f-be97-350b6f4326fb" xlink:to="loc_us-gaap_AccountsReceivableMember_019edbd5-e61b-7523-8adf-f64f7d63be9b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019edbd5-e61b-7564-a937-0e38098d0825" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-72d8-9e0b-79e43315f285" xlink:to="loc_srt_RangeAxis_019edbd5-e61b-7564-a937-0e38098d0825" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019edbd5-e61b-7564-a937-0e38098d0825_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019edbd5-e61b-7564-a937-0e38098d0825" xlink:to="loc_srt_RangeMember_019edbd5-e61b-7564-a937-0e38098d0825_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019edbd5-e61b-7bd4-825c-e6da2cef0823" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019edbd5-e61b-7564-a937-0e38098d0825" xlink:to="loc_srt_RangeMember_019edbd5-e61b-7bd4-825c-e6da2cef0823" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019edbd5-e61b-7036-b7b0-2e536c63b73f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019edbd5-e61b-7bd4-825c-e6da2cef0823" xlink:to="loc_srt_MinimumMember_019edbd5-e61b-7036-b7b0-2e536c63b73f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019edbd5-e61b-74c4-8155-1cc40cef1f2b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-72d8-9e0b-79e43315f285" xlink:to="loc_srt_CounterpartyNameAxis_019edbd5-e61b-74c4-8155-1cc40cef1f2b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019edbd5-e61b-74c4-8155-1cc40cef1f2b_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019edbd5-e61b-74c4-8155-1cc40cef1f2b" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019edbd5-e61b-74c4-8155-1cc40cef1f2b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019edbd5-e61b-78fb-a3be-5f6c01dc86f7" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019edbd5-e61b-74c4-8155-1cc40cef1f2b" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019edbd5-e61b-78fb-a3be-5f6c01dc86f7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXMember_019edbd5-e61b-7619-a969-526a0d869ca9" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019edbd5-e61b-78fb-a3be-5f6c01dc86f7" xlink:to="loc_ck1437958_CCBXMember_019edbd5-e61b-7619-a969-526a0d869ca9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralAxis_019edbd5-e61b-7a15-b6b5-a90afb44179d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-72d8-9e0b-79e43315f285" xlink:to="loc_us-gaap_CollateralAxis_019edbd5-e61b-7a15-b6b5-a90afb44179d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralDomain_019edbd5-e61b-7a15-b6b5-a90afb44179d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CollateralAxis_019edbd5-e61b-7a15-b6b5-a90afb44179d" xlink:to="loc_us-gaap_CollateralDomain_019edbd5-e61b-7a15-b6b5-a90afb44179d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralDomain_019edbd5-e61b-748f-ba09-fed16b70ddbd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CollateralAxis_019edbd5-e61b-7a15-b6b5-a90afb44179d" xlink:to="loc_us-gaap_CollateralDomain_019edbd5-e61b-748f-ba09-fed16b70ddbd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_OwnerOccupiedRealEstateMember_019edbd5-e61b-714d-9091-27ffbf880f4e" xlink:href="ck1437958-20260630.xsd#ck1437958_OwnerOccupiedRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019edbd5-e61b-748f-ba09-fed16b70ddbd" xlink:to="loc_ck1437958_OwnerOccupiedRealEstateMember_019edbd5-e61b-714d-9091-27ffbf880f4e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_NonOwnerOccupiedRealEstateMember_019edbd5-e61b-7480-9464-52ba030e3e06" xlink:href="ck1437958-20260630.xsd#ck1437958_NonOwnerOccupiedRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019edbd5-e61b-748f-ba09-fed16b70ddbd" xlink:to="loc_ck1437958_NonOwnerOccupiedRealEstateMember_019edbd5-e61b-7480-9464-52ba030e3e06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_MultiFamilyResidentialLoansMember_019edbd5-e61b-7801-8189-021a1f23576c" xlink:href="ck1437958-20260630.xsd#ck1437958_MultiFamilyResidentialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019edbd5-e61b-748f-ba09-fed16b70ddbd" xlink:to="loc_ck1437958_MultiFamilyResidentialLoansMember_019edbd5-e61b-7801-8189-021a1f23576c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_FarmlandLoansMember_019edbd5-e61b-7e3a-84f1-221fb98be697" xlink:href="ck1437958-20260630.xsd#ck1437958_FarmlandLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019edbd5-e61b-748f-ba09-fed16b70ddbd" xlink:to="loc_ck1437958_FarmlandLoansMember_019edbd5-e61b-7e3a-84f1-221fb98be697" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019edbd5-e61b-7e85-ba19-13b04091856b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablesPeriodPastDueAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-72d8-9e0b-79e43315f285" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019edbd5-e61b-7e85-ba19-13b04091856b" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7e85-ba19-13b04091856b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablesPeriodPastDueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019edbd5-e61b-7e85-ba19-13b04091856b" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7e85-ba19-13b04091856b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7d89-bb4a-35deac6b95ae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablesPeriodPastDueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019edbd5-e61b-7e85-ba19-13b04091856b" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7d89-bb4a-35deac6b95ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_LessThan90DaysPastDueMember_019edbd5-e61b-705f-a36f-021fba335c80" xlink:href="ck1437958-20260630.xsd#ck1437958_LessThan90DaysPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7d89-bb4a-35deac6b95ae" xlink:to="loc_ck1437958_LessThan90DaysPastDueMember_019edbd5-e61b-705f-a36f-021fba335c80" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-747c-8785-435ad85dae77" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-72d8-9e0b-79e43315f285" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-747c-8785-435ad85dae77" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e61b-747c-8785-435ad85dae77_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-747c-8785-435ad85dae77" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e61b-747c-8785-435ad85dae77_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e61b-7235-871f-455b10dc8e93" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-747c-8785-435ad85dae77" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e61b-7235-871f-455b10dc8e93" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXSegmentMember_019edbd5-e61b-7031-a207-27102a02a279" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e61b-7235-871f-455b10dc8e93" xlink:to="loc_ck1437958_CCBXSegmentMember_019edbd5-e61b-7031-a207-27102a02a279" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommunityBankSegmentMember_019edbd5-e61b-7755-8d89-68e06ba27796" xlink:href="ck1437958-20260630.xsd#ck1437958_CommunityBankSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e61b-7235-871f-455b10dc8e93" xlink:to="loc_ck1437958_CommunityBankSegmentMember_019edbd5-e61b-7755-8d89-68e06ba27796" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSaleOfLoansHeldForSale_019edbd5-e61b-73a8-a87a-8f98c2039bd4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromSaleOfLoansHeldForSale"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_us-gaap_ProceedsFromSaleOfLoansHeldForSale_019edbd5-e61b-73a8-a87a-8f98c2039bd4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferOfLoansHeldForSaleToPortfolioLoans1_019edbd5-e61b-7477-abf5-a1c60bd2d92b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransferOfLoansHeldForSaleToPortfolioLoans1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_us-gaap_TransferOfLoansHeldForSaleToPortfolioLoans1_019edbd5-e61b-7477-abf5-a1c60bd2d92b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansReceivableHeldForSaleAmount_019edbd5-e61b-70ca-9bb7-1108f7e10843" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansReceivableHeldForSaleAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_us-gaap_LoansReceivableHeldForSaleAmount_019edbd5-e61b-70ca-9bb7-1108f7e10843" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-7d69-b196-8f2a0f8231ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableAccruedInterestBeforeAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_us-gaap_FinancingReceivableAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-7d69-b196-8f2a0f8231ff" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterestNoncurrent_019edbd5-e61b-705e-99aa-305df5636c9d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterestNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterestNoncurrent_019edbd5-e61b-705e-99aa-305df5636c9d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CapitalCallAmount_019edbd5-e61b-7975-b511-8d75055eaa38" xlink:href="ck1437958-20260630.xsd#ck1437958_CapitalCallAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_CapitalCallAmount_019edbd5-e61b-7975-b511-8d75055eaa38" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BankOverdrafts_019edbd5-e61b-78ec-a5e0-aa9138fcec31" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BankOverdrafts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_us-gaap_BankOverdrafts_019edbd5-e61b-78ec-a5e0-aa9138fcec31" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestAfterAllowanceForCreditLoss_019edbd5-e61b-7ddf-95af-efd8aecec617" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestAfterAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestAfterAllowanceForCreditLoss_019edbd5-e61b-7ddf-95af-efd8aecec617" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_SmallBusinessAdministrationAndUnitedStatesDepartmentOfAgricultureLoansAndParticipationsSoldAndServiced_019edbd5-e61b-71e0-9944-3dc43e6db8d9" xlink:href="ck1437958-20260630.xsd#ck1437958_SmallBusinessAdministrationAndUnitedStatesDepartmentOfAgricultureLoansAndParticipationsSoldAndServiced"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_SmallBusinessAdministrationAndUnitedStatesDepartmentOfAgricultureLoansAndParticipationsSoldAndServiced_019edbd5-e61b-71e0-9944-3dc43e6db8d9" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_MainStreetLendingProgramLoansAndParticipationsServiced_019edbd5-e61b-7e71-899b-7cdf2e1d7e86" xlink:href="ck1437958-20260630.xsd#ck1437958_MainStreetLendingProgramLoansAndParticipationsServiced"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_MainStreetLendingProgramLoansAndParticipationsServiced_019edbd5-e61b-7e71-899b-7cdf2e1d7e86" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_AmountOfLoansPurchasedWithRemainingBalances_019edbd5-e61b-7dde-8af1-96ee4a2d70b4" xlink:href="ck1437958-20260630.xsd#ck1437958_AmountOfLoansPurchasedWithRemainingBalances"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_AmountOfLoansPurchasedWithRemainingBalances_019edbd5-e61b-7dde-8af1-96ee4a2d70b4" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableUnamortizedPurchasePremiumDiscount_019edbd5-e61b-7ecc-8514-ae3154840503" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableUnamortizedPurchasePremiumDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_us-gaap_FinancingReceivableUnamortizedPurchasePremiumDiscount_019edbd5-e61b-7ecc-8514-ae3154840503" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_PurchasedParticipationLoansWithRemainingBalances_019edbd5-e61b-73fd-a956-3240059b538f" xlink:href="ck1437958-20260630.xsd#ck1437958_PurchasedParticipationLoansWithRemainingBalances"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_PurchasedParticipationLoansWithRemainingBalances_019edbd5-e61b-73fd-a956-3240059b538f" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConsumerAndOtherLoans_019edbd5-e61b-73d3-b87e-cc70f23ce66f" xlink:href="ck1437958-20260630.xsd#ck1437958_ConsumerAndOtherLoans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_ConsumerAndOtherLoans_019edbd5-e61b-73d3-b87e-cc70f23ce66f" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialAndIndustrialLoans_019edbd5-e61b-7fd2-aa09-93ed3bbc90b4" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialAndIndustrialLoans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_CommercialAndIndustrialLoans_019edbd5-e61b-7fd2-aa09-93ed3bbc90b4" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ResidentialRealEstateLoans_019edbd5-e61b-7855-a734-8adaa26fc937" xlink:href="ck1437958-20260630.xsd#ck1437958_ResidentialRealEstateLoans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_ResidentialRealEstateLoans_019edbd5-e61b-7855-a734-8adaa26fc937" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialRealEstateLoans_019edbd5-e61b-7b7f-a0ea-af83202bca2f" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialRealEstateLoans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_CommercialRealEstateLoans_019edbd5-e61b-7b7f-a0ea-af83202bca2f" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterest90DaysOrMorePastDueStillAccruing_019edbd5-e61b-744b-b7ee-b17e7035aa7d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterest90DaysOrMorePastDueStillAccruing"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterest90DaysOrMorePastDueStillAccruing_019edbd5-e61b-744b-b7ee-b17e7035aa7d" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableThresholdPeriodPastDue_019edbd5-e61b-7329-b5c5-0a9788a486d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableThresholdPeriodPastDue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_us-gaap_FinancingReceivableThresholdPeriodPastDue_019edbd5-e61b-7329-b5c5-0a9788a486d8" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus_019edbd5-e61b-7e05-861e-2f71d024c6fb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus_019edbd5-e61b-7e05-861e-2f71d024c6fb" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNoncurrent90DaysOrMorePastDueStillAccruing_019edbd5-e61b-7f9b-ae39-eaba08272438" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableNoncurrent90DaysOrMorePastDueStillAccruing"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_us-gaap_AccountsReceivableNoncurrent90DaysOrMorePastDueStillAccruing_019edbd5-e61b-7f9b-ae39-eaba08272438" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_SustainedRepaymentPerformancePeriodOfLoanPlacedOnNonaccrual_019edbd5-e61b-7976-b4a2-2bd92ebaa4a3" xlink:href="ck1437958-20260630.xsd#ck1437958_SustainedRepaymentPerformancePeriodOfLoanPlacedOnNonaccrual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_SustainedRepaymentPerformancePeriodOfLoanPlacedOnNonaccrual_019edbd5-e61b-7976-b4a2-2bd92ebaa4a3" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_PercentageOfCollateralCoverageToLoanBalance_019edbd5-e61b-7a9a-9008-744481ab2b8f" xlink:href="ck1437958-20260630.xsd#ck1437958_PercentageOfCollateralCoverageToLoanBalance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_PercentageOfCollateralCoverageToLoanBalance_019edbd5-e61b-7a9a-9008-744481ab2b8f" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest_019edbd5-e61b-7d1e-9347-a830786967e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest_019edbd5-e61b-7d1e-9347-a830786967e1" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesPeriodIncreaseDecrease_019fcdb0-4b0d-7608-9636-852b4c9e52e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesPeriodIncreaseDecrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesPeriodIncreaseDecrease_019fcdb0-4b0d-7608-9636-852b4c9e52e1" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterestResponsibilityOfCompanyPercentage_019edbd5-e61b-7f8b-9137-d0a022b31215" xlink:href="ck1437958-20260630.xsd#ck1437958_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterestResponsibilityOfCompanyPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterestResponsibilityOfCompanyPercentage_019edbd5-e61b-7f8b-9137-d0a022b31215" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-7e51-9aef-7e55ddc149eb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-7e51-9aef-7e55ddc149eb" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossResponsibilityCreditLossesAmount_019edbd5-e61b-7968-9a20-9fa4e5df0690" xlink:href="ck1437958-20260630.xsd#ck1437958_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossResponsibilityCreditLossesAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossResponsibilityCreditLossesAmount_019edbd5-e61b-7968-9a20-9fa4e5df0690" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ProvisionRecaptureForUnfundedCommitments_019edbd5-e61b-7ac6-9d1e-73356801a09d" xlink:href="ck1437958-20260630.xsd#ck1437958_ProvisionRecaptureForUnfundedCommitments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_ProvisionRecaptureForUnfundedCommitments_019edbd5-e61b-7ac6-9d1e-73356801a09d" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_LoanRepaymentExtendedPeriodOfTime_019edbd5-e61b-73f6-b13c-b0f373b193dd" xlink:href="ck1437958-20260630.xsd#ck1437958_LoanRepaymentExtendedPeriodOfTime"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_LoanRepaymentExtendedPeriodOfTime_019edbd5-e61b-73f6-b13c-b0f373b193dd" xlink:type="arc" order="29"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess_019edbd5-e61b-78e1-a8e7-c7f75d316599" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess_019edbd5-e61b-78e1-a8e7-c7f75d316599" xlink:type="arc" order="30"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterestResponsibilityOfPartnerPercentage_019edbd5-e61b-73d5-84d4-f070a519e079" xlink:href="ck1437958-20260630.xsd#ck1437958_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterestResponsibilityOfPartnerPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterestResponsibilityOfPartnerPercentage_019edbd5-e61b-73d5-84d4-f070a519e079" xlink:type="arc" order="31"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossRevenueEarnedPercentage_019edbd5-e61b-7750-a6f8-1584bb694e32" xlink:href="ck1437958-20260630.xsd#ck1437958_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossRevenueEarnedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossRevenueEarnedPercentage_019edbd5-e61b-7750-a6f8-1584bb694e32" xlink:type="arc" order="32"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_FinancingReceivableFraudLossesExcludingAccruedInterestResponsibilityOfPartnerPercentage_019edbd5-e61b-70cf-b5f8-f54ed2633f0a" xlink:href="ck1437958-20260630.xsd#ck1437958_FinancingReceivableFraudLossesExcludingAccruedInterestResponsibilityOfPartnerPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-78d2-b627-fd20522e5275" xlink:to="loc_ck1437958_FinancingReceivableFraudLossesExcludingAccruedInterestResponsibilityOfPartnerPercentage_019edbd5-e61b-70cf-b5f8-f54ed2633f0a" xlink:type="arc" order="33"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesCompositionofLoanPortfolioDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#LoansandAllowanceforCreditLossesCompositionofLoanPortfolioDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesCompositionofLoanPortfolioDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsLineItems_019edbd5-e61b-75ea-9e78-4cb005462d56" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansAndLeasesReceivableDisclosureTable_019edbd5-e61b-72d1-ad83-5232a36f3551" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansAndLeasesReceivableDisclosureTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsLineItems_019edbd5-e61b-75ea-9e78-4cb005462d56" xlink:to="loc_us-gaap_LoansAndLeasesReceivableDisclosureTable_019edbd5-e61b-72d1-ad83-5232a36f3551" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-70ae-9663-ca8231d4fdc3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LoansAndLeasesReceivableDisclosureTable_019edbd5-e61b-72d1-ad83-5232a36f3551" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-70ae-9663-ca8231d4fdc3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e61b-70ae-9663-ca8231d4fdc3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-70ae-9663-ca8231d4fdc3" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e61b-70ae-9663-ca8231d4fdc3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e61b-7755-b260-c00432952fbb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-70ae-9663-ca8231d4fdc3" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e61b-7755-b260-c00432952fbb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommunityBankSegmentMember_019edbd5-e61b-7ed7-9e6f-d89e01582e34" xlink:href="ck1437958-20260630.xsd#ck1437958_CommunityBankSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e61b-7755-b260-c00432952fbb" xlink:to="loc_ck1437958_CommunityBankSegmentMember_019edbd5-e61b-7ed7-9e6f-d89e01582e34" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXSegmentMember_019edbd5-e61b-736b-b098-a5cd4d7f8a33" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e61b-7755-b260-c00432952fbb" xlink:to="loc_ck1437958_CCBXSegmentMember_019edbd5-e61b-736b-b098-a5cd4d7f8a33" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7af7-9135-7ab61ce4f746" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LoansAndLeasesReceivableDisclosureTable_019edbd5-e61b-72d1-ad83-5232a36f3551" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7af7-9135-7ab61ce4f746" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7af7-9135-7ab61ce4f746_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7af7-9135-7ab61ce4f746" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7af7-9135-7ab61ce4f746_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7183-8e14-8fb7ac557b71" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7af7-9135-7ab61ce4f746" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7183-8e14-8fb7ac557b71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPortfolioSegmentMember_019edbd5-e61b-77df-a5b5-3b60a4100c8b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7183-8e14-8fb7ac557b71" xlink:to="loc_us-gaap_CommercialPortfolioSegmentMember_019edbd5-e61b-77df-a5b5-3b60a4100c8b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember_019edbd5-e61b-77ae-99f3-2a839fc642b6" xlink:href="ck1437958-20260630.xsd#ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7183-8e14-8fb7ac557b71" xlink:to="loc_ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember_019edbd5-e61b-77ae-99f3-2a839fc642b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-7cea-8f1b-15960f2b07e7" xlink:href="ck1437958-20260630.xsd#ck1437958_RealEstatePortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7183-8e14-8fb7ac557b71" xlink:to="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-7cea-8f1b-15960f2b07e7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-7833-b5f0-79b5c88fc9af" xlink:href="ck1437958-20260630.xsd#ck1437958_ConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7183-8e14-8fb7ac557b71" xlink:to="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-7833-b5f0-79b5c88fc9af" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7c10-8029-47635a7bf331" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LoansAndLeasesReceivableDisclosureTable_019edbd5-e61b-72d1-ad83-5232a36f3551" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7c10-8029-47635a7bf331" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7c10-8029-47635a7bf331_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7c10-8029-47635a7bf331" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7c10-8029-47635a7bf331_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-74bb-9a37-529ab6c65f7b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7c10-8029-47635a7bf331" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-74bb-9a37-529ab6c65f7b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-790d-bb83-65d473d4c26b" xlink:href="ck1437958-20260630.xsd#ck1437958_ResidentialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-74bb-9a37-529ab6c65f7b" xlink:to="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-790d-bb83-65d473d4c26b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialRealEstateLoansMember_019edbd5-e61b-7dea-a93c-a01d93fb1cdc" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-74bb-9a37-529ab6c65f7b" xlink:to="loc_ck1437958_CommercialRealEstateLoansMember_019edbd5-e61b-7dea-a93c-a01d93fb1cdc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXCapitalCallLinesMember_019edbd5-e61b-7a3c-b783-12415ba53c48" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXCapitalCallLinesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-74bb-9a37-529ab6c65f7b" xlink:to="loc_ck1437958_CCBXCapitalCallLinesMember_019edbd5-e61b-7a3c-b783-12415ba53c48" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_OtherCommercialAndIndustrialLoansMember_019edbd5-e61b-7f61-ac8d-f9983d699543" xlink:href="ck1437958-20260630.xsd#ck1437958_OtherCommercialAndIndustrialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-74bb-9a37-529ab6c65f7b" xlink:to="loc_ck1437958_OtherCommercialAndIndustrialLoansMember_019edbd5-e61b-7f61-ac8d-f9983d699543" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditCardMember_019edbd5-e61b-7e2f-97cd-411f26de65cf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditCardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-74bb-9a37-529ab6c65f7b" xlink:to="loc_us-gaap_CreditCardMember_019edbd5-e61b-7e2f-97cd-411f26de65cf" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess_019edbd5-e61b-7419-9a64-2c245a9db67a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsLineItems_019edbd5-e61b-75ea-9e78-4cb005462d56" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess_019edbd5-e61b-7419-9a64-2c245a9db67a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount_019edbd5-e61b-7919-8aea-6d5cbad0238a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsLineItems_019edbd5-e61b-75ea-9e78-4cb005462d56" xlink:to="loc_us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount_019edbd5-e61b-7919-8aea-6d5cbad0238a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-703a-89a4-ae3df0be46c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsLineItems_019edbd5-e61b-75ea-9e78-4cb005462d56" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-703a-89a4-ae3df0be46c7" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesBreakdownofConsumerLoanPortfoliobySegmentandTypeofLoanDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#LoansandAllowanceforCreditLossesBreakdownofConsumerLoanPortfoliobySegmentandTypeofLoanDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesBreakdownofConsumerLoanPortfoliobySegmentandTypeofLoanDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-765a-ba5c-6d151446bed7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-7862-909d-fa9855741705" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-765a-ba5c-6d151446bed7" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-7862-909d-fa9855741705" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-72bd-808d-50b712812c9f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-7862-909d-fa9855741705" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-72bd-808d-50b712812c9f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-72bd-808d-50b712812c9f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-72bd-808d-50b712812c9f" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-72bd-808d-50b712812c9f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-782f-8140-4643c0835795" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-72bd-808d-50b712812c9f" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-782f-8140-4643c0835795" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-7df2-ae7a-3531bef93fb1" xlink:href="ck1437958-20260630.xsd#ck1437958_ConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-782f-8140-4643c0835795" xlink:to="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-7df2-ae7a-3531bef93fb1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7007-a402-3885111b607a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-7862-909d-fa9855741705" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7007-a402-3885111b607a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e61b-7007-a402-3885111b607a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7007-a402-3885111b607a" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e61b-7007-a402-3885111b607a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e61b-7d8c-a4ed-7effd7ccaf44" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7007-a402-3885111b607a" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e61b-7d8c-a4ed-7effd7ccaf44" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXSegmentMember_019edbd5-e61b-7704-aee7-365e56814a0a" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e61b-7d8c-a4ed-7effd7ccaf44" xlink:to="loc_ck1437958_CCBXSegmentMember_019edbd5-e61b-7704-aee7-365e56814a0a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommunityBankSegmentMember_019edbd5-e61b-7fcd-b1f4-95cefeb8fb85" xlink:href="ck1437958-20260630.xsd#ck1437958_CommunityBankSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e61b-7d8c-a4ed-7effd7ccaf44" xlink:to="loc_ck1437958_CommunityBankSegmentMember_019edbd5-e61b-7fcd-b1f4-95cefeb8fb85" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7880-b945-cf7ad7d24fc6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-7862-909d-fa9855741705" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7880-b945-cf7ad7d24fc6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7880-b945-cf7ad7d24fc6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7880-b945-cf7ad7d24fc6" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7880-b945-cf7ad7d24fc6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-760a-83ca-19ff5ed88d17" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7880-b945-cf7ad7d24fc6" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-760a-83ca-19ff5ed88d17" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_InstallmentLoansCashSecuredMember_019edbd5-e61b-77ae-956e-2a51a666d9ca" xlink:href="ck1437958-20260630.xsd#ck1437958_InstallmentLoansCashSecuredMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-760a-83ca-19ff5ed88d17" xlink:to="loc_ck1437958_InstallmentLoansCashSecuredMember_019edbd5-e61b-77ae-956e-2a51a666d9ca" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_InstallmentLoansUnsecuredMember_019edbd5-e61b-7163-b2a9-5321fbfceb58" xlink:href="ck1437958-20260630.xsd#ck1437958_InstallmentLoansUnsecuredMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-760a-83ca-19ff5ed88d17" xlink:to="loc_ck1437958_InstallmentLoansUnsecuredMember_019edbd5-e61b-7163-b2a9-5321fbfceb58" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_InstallmentLoansTotalMember_019edbd5-e61b-77d2-a7e8-2700dc4cb545" xlink:href="ck1437958-20260630.xsd#ck1437958_InstallmentLoansTotalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-760a-83ca-19ff5ed88d17" xlink:to="loc_ck1437958_InstallmentLoansTotalMember_019edbd5-e61b-77d2-a7e8-2700dc4cb545" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CreditCardsCashSecuredMember_019edbd5-e61b-7046-98e1-85e3ef0b3989" xlink:href="ck1437958-20260630.xsd#ck1437958_CreditCardsCashSecuredMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-760a-83ca-19ff5ed88d17" xlink:to="loc_ck1437958_CreditCardsCashSecuredMember_019edbd5-e61b-7046-98e1-85e3ef0b3989" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CreditCardsUnsecuredMember_019edbd5-e61b-7b0d-ba8f-eec5efb5e296" xlink:href="ck1437958-20260630.xsd#ck1437958_CreditCardsUnsecuredMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-760a-83ca-19ff5ed88d17" xlink:to="loc_ck1437958_CreditCardsUnsecuredMember_019edbd5-e61b-7b0d-ba8f-eec5efb5e296" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CreditCardsTotalMember_019edbd5-e61b-78fc-b758-38d66e47286b" xlink:href="ck1437958-20260630.xsd#ck1437958_CreditCardsTotalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-760a-83ca-19ff5ed88d17" xlink:to="loc_ck1437958_CreditCardsTotalMember_019edbd5-e61b-78fc-b758-38d66e47286b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_LinesOfCreditMember_019edbd5-e61b-75bb-bf62-2132ea890c00" xlink:href="ck1437958-20260630.xsd#ck1437958_LinesOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-760a-83ca-19ff5ed88d17" xlink:to="loc_ck1437958_LinesOfCreditMember_019edbd5-e61b-75bb-bf62-2132ea890c00" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConsumerOtherMember_019edbd5-e61b-7036-ba26-83aeef9f07b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConsumerOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-760a-83ca-19ff5ed88d17" xlink:to="loc_us-gaap_ConsumerOtherMember_019edbd5-e61b-7036-ba26-83aeef9f07b7" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess_019edbd5-e61b-7c8c-917e-be355e6b0138" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-765a-ba5c-6d151446bed7" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess_019edbd5-e61b-7c8c-917e-be355e6b0138" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_FinancingReceivableBeforeAllowanceForCreditLossToTotalPercent_019edbd5-e61b-7ca5-b100-76e9ef5edd34" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_FinancingReceivableBeforeAllowanceForCreditLossToTotalPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-765a-ba5c-6d151446bed7" xlink:to="loc_srt_FinancingReceivableBeforeAllowanceForCreditLossToTotalPercent_019edbd5-e61b-7ca5-b100-76e9ef5edd34" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesBeforeAllowanceForCreditLossAverageAmountOutstanding_019edbd5-e61b-7710-a587-88b8a60be41b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablesBeforeAllowanceForCreditLossAverageAmountOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-765a-ba5c-6d151446bed7" xlink:to="loc_us-gaap_FinancingReceivablesBeforeAllowanceForCreditLossAverageAmountOutstanding_019edbd5-e61b-7710-a587-88b8a60be41b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_FinancingReceivableBeforeAllowanceForCreditLossOutstandingNumberOfLoans_019edbd5-e61b-7bb6-af30-59584081d72d" xlink:href="ck1437958-20260630.xsd#ck1437958_FinancingReceivableBeforeAllowanceForCreditLossOutstandingNumberOfLoans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-765a-ba5c-6d151446bed7" xlink:to="loc_ck1437958_FinancingReceivableBeforeAllowanceForCreditLossOutstandingNumberOfLoans_019edbd5-e61b-7bb6-af30-59584081d72d" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesSummaryofanAgeAnalysisofPastDueLoansDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#LoansandAllowanceforCreditLossesSummaryofanAgeAnalysisofPastDueLoansDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesSummaryofanAgeAnalysisofPastDueLoansDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019edbd5-e61b-7181-9c42-64fd6e041e4d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019edbd5-e61b-71df-a6ef-e8678de3aac4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019edbd5-e61b-7181-9c42-64fd6e041e4d" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019edbd5-e61b-71df-a6ef-e8678de3aac4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7e3d-9f6f-524e1635a6fa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019edbd5-e61b-71df-a6ef-e8678de3aac4" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7e3d-9f6f-524e1635a6fa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e61b-7e3d-9f6f-524e1635a6fa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7e3d-9f6f-524e1635a6fa" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e61b-7e3d-9f6f-524e1635a6fa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e61b-74e1-86c4-2baeec4f0b6f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7e3d-9f6f-524e1635a6fa" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e61b-74e1-86c4-2baeec4f0b6f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommunityBankSegmentMember_019edbd5-e61b-73a8-92ec-42dc22dd6bfd" xlink:href="ck1437958-20260630.xsd#ck1437958_CommunityBankSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e61b-74e1-86c4-2baeec4f0b6f" xlink:to="loc_ck1437958_CommunityBankSegmentMember_019edbd5-e61b-73a8-92ec-42dc22dd6bfd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXSegmentMember_019edbd5-e61b-7e12-a8db-21c6b03b2f2e" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e61b-74e1-86c4-2baeec4f0b6f" xlink:to="loc_ck1437958_CCBXSegmentMember_019edbd5-e61b-7e12-a8db-21c6b03b2f2e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7eaa-8480-9a4c543b85f9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019edbd5-e61b-71df-a6ef-e8678de3aac4" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7eaa-8480-9a4c543b85f9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7eaa-8480-9a4c543b85f9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7eaa-8480-9a4c543b85f9" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7eaa-8480-9a4c543b85f9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-70e3-812a-891df4480b4a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7eaa-8480-9a4c543b85f9" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-70e3-812a-891df4480b4a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPortfolioSegmentMember_019edbd5-e61b-7e2e-b64a-1603f8691d3c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-70e3-812a-891df4480b4a" xlink:to="loc_us-gaap_CommercialPortfolioSegmentMember_019edbd5-e61b-7e2e-b64a-1603f8691d3c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember_019edbd5-e61b-760a-af71-d3e66f549d03" xlink:href="ck1437958-20260630.xsd#ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-70e3-812a-891df4480b4a" xlink:to="loc_ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember_019edbd5-e61b-760a-af71-d3e66f549d03" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-760c-8d63-f88f4a5af567" xlink:href="ck1437958-20260630.xsd#ck1437958_RealEstatePortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-70e3-812a-891df4480b4a" xlink:to="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-760c-8d63-f88f4a5af567" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-7251-a402-3837b03fc767" xlink:href="ck1437958-20260630.xsd#ck1437958_ConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-70e3-812a-891df4480b4a" xlink:to="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-7251-a402-3837b03fc767" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019edbd5-e61b-7336-b64a-29c83bae5880" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablesPeriodPastDueAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019edbd5-e61b-71df-a6ef-e8678de3aac4" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019edbd5-e61b-7336-b64a-29c83bae5880" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7336-b64a-29c83bae5880_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablesPeriodPastDueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019edbd5-e61b-7336-b64a-29c83bae5880" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7336-b64a-29c83bae5880_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7de8-8d96-89c61a069ec8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablesPeriodPastDueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019edbd5-e61b-7336-b64a-29c83bae5880" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7de8-8d96-89c61a069ec8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_FinancialAsset30To89DaysPastDueMember_019edbd5-e61b-7a2d-a639-915d83df95e5" xlink:href="ck1437958-20260630.xsd#ck1437958_FinancialAsset30To89DaysPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7de8-8d96-89c61a069ec8" xlink:to="loc_ck1437958_FinancialAsset30To89DaysPastDueMember_019edbd5-e61b-7a2d-a639-915d83df95e5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember_019edbd5-e61b-74bb-b00d-ffcea5b843a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7de8-8d96-89c61a069ec8" xlink:to="loc_us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember_019edbd5-e61b-74bb-b00d-ffcea5b843a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialAssetPastDueMember_019edbd5-e61b-7135-b20c-da9840832547" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialAssetPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7de8-8d96-89c61a069ec8" xlink:to="loc_us-gaap_FinancialAssetPastDueMember_019edbd5-e61b-7135-b20c-da9840832547" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialAssetNotPastDueMember_019edbd5-e61b-7acf-82c2-da8b28f0f1f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialAssetNotPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7de8-8d96-89c61a069ec8" xlink:to="loc_us-gaap_FinancialAssetNotPastDueMember_019edbd5-e61b-7acf-82c2-da8b28f0f1f6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-740f-9e0f-3ed8a2d78697" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019edbd5-e61b-71df-a6ef-e8678de3aac4" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-740f-9e0f-3ed8a2d78697" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-740f-9e0f-3ed8a2d78697_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-740f-9e0f-3ed8a2d78697" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-740f-9e0f-3ed8a2d78697_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7962-82f0-0013a85eb7f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-740f-9e0f-3ed8a2d78697" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7962-82f0-0013a85eb7f4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-759f-8043-fb5202f3200b" xlink:href="ck1437958-20260630.xsd#ck1437958_ResidentialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7962-82f0-0013a85eb7f4" xlink:to="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-759f-8043-fb5202f3200b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialRealEstateLoansMember_019edbd5-e61b-74f4-8505-591b2d937fea" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7962-82f0-0013a85eb7f4" xlink:to="loc_ck1437958_CommercialRealEstateLoansMember_019edbd5-e61b-74f4-8505-591b2d937fea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXCapitalCallLinesMember_019edbd5-e61b-7bb4-a72e-7915fb1bcf11" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXCapitalCallLinesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7962-82f0-0013a85eb7f4" xlink:to="loc_ck1437958_CCBXCapitalCallLinesMember_019edbd5-e61b-7bb4-a72e-7915fb1bcf11" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXOtherCommercialAndIndustrialLoansMember_019edbd5-e61b-79b6-a8d6-221dc7c374db" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXOtherCommercialAndIndustrialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7962-82f0-0013a85eb7f4" xlink:to="loc_ck1437958_CCBXOtherCommercialAndIndustrialLoansMember_019edbd5-e61b-79b6-a8d6-221dc7c374db" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditCardMember_019edbd5-e61b-776a-bf97-10b223ccbda9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditCardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7962-82f0-0013a85eb7f4" xlink:to="loc_us-gaap_CreditCardMember_019edbd5-e61b-776a-bf97-10b223ccbda9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_OtherConsumerAndOtherLoansMember_019edbd5-e61b-711d-ac1b-69a31712acf3" xlink:href="ck1437958-20260630.xsd#ck1437958_OtherConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7962-82f0-0013a85eb7f4" xlink:to="loc_ck1437958_OtherConsumerAndOtherLoansMember_019edbd5-e61b-711d-ac1b-69a31712acf3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess_019edbd5-e61b-72fb-9e19-f9094838f911" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019edbd5-e61b-7181-9c42-64fd6e041e4d" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess_019edbd5-e61b-72fb-9e19-f9094838f911" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterest90DaysOrMorePastDueStillAccruing_019edbd5-e61b-75e8-a0c3-6624a8ccb6dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterest90DaysOrMorePastDueStillAccruing"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019edbd5-e61b-7181-9c42-64fd6e041e4d" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterest90DaysOrMorePastDueStillAccruing_019edbd5-e61b-75e8-a0c3-6624a8ccb6dc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount_019edbd5-e61b-775f-9250-c05cbbddd025" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019edbd5-e61b-7181-9c42-64fd6e041e4d" xlink:to="loc_us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount_019edbd5-e61b-775f-9250-c05cbbddd025" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-70e1-b239-a27bfe811901" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019edbd5-e61b-7181-9c42-64fd6e041e4d" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-70e1-b239-a27bfe811901" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesAnalysisofNonaccrualLoansbyCategoryDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#LoansandAllowanceforCreditLossesAnalysisofNonaccrualLoansbyCategoryDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesAnalysisofNonaccrualLoansbyCategoryDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019edbd5-e61b-7967-b2f2-5cf871a0c494" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019edbd5-e61b-7124-ab05-c2608640e63c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfFinancingReceivablesPastDueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019edbd5-e61b-7967-b2f2-5cf871a0c494" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019edbd5-e61b-7124-ab05-c2608640e63c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7e92-9d6f-5a88d5cd1fd4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019edbd5-e61b-7124-ab05-c2608640e63c" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7e92-9d6f-5a88d5cd1fd4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e61b-7e92-9d6f-5a88d5cd1fd4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7e92-9d6f-5a88d5cd1fd4" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e61b-7e92-9d6f-5a88d5cd1fd4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e61b-71b3-969f-a51b5e28f847" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7e92-9d6f-5a88d5cd1fd4" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e61b-71b3-969f-a51b5e28f847" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommunityBankSegmentMember_019edbd5-e61b-73a6-a720-4614f9e46c30" xlink:href="ck1437958-20260630.xsd#ck1437958_CommunityBankSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e61b-71b3-969f-a51b5e28f847" xlink:to="loc_ck1437958_CommunityBankSegmentMember_019edbd5-e61b-73a6-a720-4614f9e46c30" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXMember_019edbd5-e61b-76a7-a1fb-e29a874aa94d" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e61b-71b3-969f-a51b5e28f847" xlink:to="loc_ck1437958_CCBXMember_019edbd5-e61b-76a7-a1fb-e29a874aa94d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-70fe-8286-b1957fec43e3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019edbd5-e61b-7124-ab05-c2608640e63c" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-70fe-8286-b1957fec43e3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-70fe-8286-b1957fec43e3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-70fe-8286-b1957fec43e3" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-70fe-8286-b1957fec43e3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7c95-923a-1704345405fb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-70fe-8286-b1957fec43e3" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7c95-923a-1704345405fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019edbd5-e61b-7a3a-8484-be2e7712ee8b" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7c95-923a-1704345405fb" xlink:to="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019edbd5-e61b-7a3a-8484-be2e7712ee8b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-79f1-855f-cd44144dce62" xlink:href="ck1437958-20260630.xsd#ck1437958_RealEstatePortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7c95-923a-1704345405fb" xlink:to="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-79f1-855f-cd44144dce62" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-7e26-ba02-901cc583794b" xlink:href="ck1437958-20260630.xsd#ck1437958_ConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7c95-923a-1704345405fb" xlink:to="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-7e26-ba02-901cc583794b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7f44-9258-b4c8b4cc32d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivablesPastDueTable_019edbd5-e61b-7124-ab05-c2608640e63c" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7f44-9258-b4c8b4cc32d1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7f44-9258-b4c8b4cc32d1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7f44-9258-b4c8b4cc32d1" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7f44-9258-b4c8b4cc32d1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7ef9-9ad2-e70cbf12e08c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7f44-9258-b4c8b4cc32d1" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7ef9-9ad2-e70cbf12e08c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-7ea9-b3f8-999c1fc6f0fa" xlink:href="ck1437958-20260630.xsd#ck1437958_ResidentialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7ef9-9ad2-e70cbf12e08c" xlink:to="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-7ea9-b3f8-999c1fc6f0fa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialRealEstateLoansMember_019edbd5-e61b-7a91-9135-0f9d3c4a61ad" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7ef9-9ad2-e70cbf12e08c" xlink:to="loc_ck1437958_CommercialRealEstateLoansMember_019edbd5-e61b-7a91-9135-0f9d3c4a61ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditCardMember_019edbd5-e61b-7a13-98f6-82e03d50e6a6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditCardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7ef9-9ad2-e70cbf12e08c" xlink:to="loc_us-gaap_CreditCardMember_019edbd5-e61b-7a13-98f6-82e03d50e6a6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConsumerAndOtherConsumerLoansMember_019edbd5-e61b-7679-abae-03025be10778" xlink:href="ck1437958-20260630.xsd#ck1437958_ConsumerAndOtherConsumerLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7ef9-9ad2-e70cbf12e08c" xlink:to="loc_ck1437958_ConsumerAndOtherConsumerLoansMember_019edbd5-e61b-7679-abae-03025be10778" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrual_019edbd5-e61b-7155-af5f-3e63aea95003" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019edbd5-e61b-7967-b2f2-5cf871a0c494" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrual_019edbd5-e61b-7155-af5f-3e63aea95003" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrualNoAllowance_019edbd5-e61b-709f-ad66-f1afb217ecdc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrualNoAllowance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019edbd5-e61b-7967-b2f2-5cf871a0c494" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestNonaccrualNoAllowance_019edbd5-e61b-709f-ad66-f1afb217ecdc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_FinancingReceivableExcludingAccruedInterestNonaccrualWithAllowance_019edbd5-e61b-711f-9192-ed7fc57ccc65" xlink:href="ck1437958-20260630.xsd#ck1437958_FinancingReceivableExcludingAccruedInterestNonaccrualWithAllowance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems_019edbd5-e61b-7967-b2f2-5cf871a0c494" xlink:to="loc_ck1437958_FinancingReceivableExcludingAccruedInterestNonaccrualWithAllowance_019edbd5-e61b-711f-9192-ed7fc57ccc65" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesLoansExperiencingFinancialDifficultyandModifiedDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#LoansandAllowanceforCreditLossesLoansExperiencingFinancialDifficultyandModifiedDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesLoansExperiencingFinancialDifficultyandModifiedDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableModificationsLineItems_019edbd5-e61b-7e0c-bdfa-010335d9a23a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableModificationsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivableTroubledDebtRestructuringsTable_019edbd5-e61b-7fb0-9ea9-0226902e4813" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfFinancingReceivableTroubledDebtRestructuringsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableModificationsLineItems_019edbd5-e61b-7e0c-bdfa-010335d9a23a" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivableTroubledDebtRestructuringsTable_019edbd5-e61b-7fb0-9ea9-0226902e4813" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7d2c-aaa0-dd9a87ca220b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableTroubledDebtRestructuringsTable_019edbd5-e61b-7fb0-9ea9-0226902e4813" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7d2c-aaa0-dd9a87ca220b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e61b-7d2c-aaa0-dd9a87ca220b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7d2c-aaa0-dd9a87ca220b" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e61b-7d2c-aaa0-dd9a87ca220b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e61b-76e9-88c0-514b37302faf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7d2c-aaa0-dd9a87ca220b" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e61b-76e9-88c0-514b37302faf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommunityBankSegmentMember_019edbd5-e61b-77ae-bca7-bcf33d75ce0f" xlink:href="ck1437958-20260630.xsd#ck1437958_CommunityBankSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e61b-76e9-88c0-514b37302faf" xlink:to="loc_ck1437958_CommunityBankSegmentMember_019edbd5-e61b-77ae-bca7-bcf33d75ce0f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXSegmentMember_019edbd5-e61b-7cae-9640-a5e22fdabbc4" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e61b-76e9-88c0-514b37302faf" xlink:to="loc_ck1437958_CCBXSegmentMember_019edbd5-e61b-7cae-9640-a5e22fdabbc4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoanRestructuringModificationAxis_019edbd5-e61b-769d-84d9-7035bd7be1ae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoanRestructuringModificationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableTroubledDebtRestructuringsTable_019edbd5-e61b-7fb0-9ea9-0226902e4813" xlink:to="loc_us-gaap_LoanRestructuringModificationAxis_019edbd5-e61b-769d-84d9-7035bd7be1ae" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoanRestructuringModificationDomain_019edbd5-e61b-769d-84d9-7035bd7be1ae_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoanRestructuringModificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LoanRestructuringModificationAxis_019edbd5-e61b-769d-84d9-7035bd7be1ae" xlink:to="loc_us-gaap_LoanRestructuringModificationDomain_019edbd5-e61b-769d-84d9-7035bd7be1ae_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoanRestructuringModificationDomain_019edbd5-e61b-7841-9c17-8013a6a50bb2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoanRestructuringModificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LoanRestructuringModificationAxis_019edbd5-e61b-769d-84d9-7035bd7be1ae" xlink:to="loc_us-gaap_LoanRestructuringModificationDomain_019edbd5-e61b-7841-9c17-8013a6a50bb2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrincipalForgivenessMember_019edbd5-e61b-7dd6-83ae-7caf37664ce5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrincipalForgivenessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoanRestructuringModificationDomain_019edbd5-e61b-7841-9c17-8013a6a50bb2" xlink:to="loc_us-gaap_PrincipalForgivenessMember_019edbd5-e61b-7dd6-83ae-7caf37664ce5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ExtendedMaturityMember_019edbd5-e61b-7875-85dc-4d04d2479ad4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ExtendedMaturityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoanRestructuringModificationDomain_019edbd5-e61b-7841-9c17-8013a6a50bb2" xlink:to="loc_us-gaap_ExtendedMaturityMember_019edbd5-e61b-7875-85dc-4d04d2479ad4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractualInterestRateReductionMember_019edbd5-e61b-7a24-b244-4ae7fdf51c76" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractualInterestRateReductionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoanRestructuringModificationDomain_019edbd5-e61b-7841-9c17-8013a6a50bb2" xlink:to="loc_us-gaap_ContractualInterestRateReductionMember_019edbd5-e61b-7a24-b244-4ae7fdf51c76" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ExtendedMaturityAndPrincipalForgivenessMember_019edbd5-e61b-7bdf-b06d-b67595f7de09" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ExtendedMaturityAndPrincipalForgivenessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoanRestructuringModificationDomain_019edbd5-e61b-7841-9c17-8013a6a50bb2" xlink:to="loc_us-gaap_ExtendedMaturityAndPrincipalForgivenessMember_019edbd5-e61b-7bdf-b06d-b67595f7de09" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_InterestRateReductionAndPaymentDelayMember_019edbd5-e61b-7d16-b9c7-6164bc4e7a18" xlink:href="ck1437958-20260630.xsd#ck1437958_InterestRateReductionAndPaymentDelayMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoanRestructuringModificationDomain_019edbd5-e61b-7841-9c17-8013a6a50bb2" xlink:to="loc_ck1437958_InterestRateReductionAndPaymentDelayMember_019edbd5-e61b-7d16-b9c7-6164bc4e7a18" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ExtendedMaturityPrincipalForgivenessAndPaymentDeferralMember_019edbd5-e61b-7acf-81c9-58d14c5dca28" xlink:href="ck1437958-20260630.xsd#ck1437958_ExtendedMaturityPrincipalForgivenessAndPaymentDeferralMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoanRestructuringModificationDomain_019edbd5-e61b-7841-9c17-8013a6a50bb2" xlink:to="loc_ck1437958_ExtendedMaturityPrincipalForgivenessAndPaymentDeferralMember_019edbd5-e61b-7acf-81c9-58d14c5dca28" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ExtendedMaturityAndInterestRateReductionMember_019edbd5-e61b-75ff-aa5b-a6970dec2a21" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ExtendedMaturityAndInterestRateReductionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoanRestructuringModificationDomain_019edbd5-e61b-7841-9c17-8013a6a50bb2" xlink:to="loc_us-gaap_ExtendedMaturityAndInterestRateReductionMember_019edbd5-e61b-75ff-aa5b-a6970dec2a21" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7204-98c8-364a206ed096" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableTroubledDebtRestructuringsTable_019edbd5-e61b-7fb0-9ea9-0226902e4813" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7204-98c8-364a206ed096" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7204-98c8-364a206ed096_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7204-98c8-364a206ed096" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7204-98c8-364a206ed096_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7824-a9b8-374c8f84fe2b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7204-98c8-364a206ed096" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7824-a9b8-374c8f84fe2b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPortfolioSegmentMember_019edbd5-e61b-74c2-b1cd-ed52cfe36749" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7824-a9b8-374c8f84fe2b" xlink:to="loc_us-gaap_CommercialPortfolioSegmentMember_019edbd5-e61b-74c2-b1cd-ed52cfe36749" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-75fc-8897-a7535d7d8e50" xlink:href="ck1437958-20260630.xsd#ck1437958_RealEstatePortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7824-a9b8-374c8f84fe2b" xlink:to="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-75fc-8897-a7535d7d8e50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-7f95-993c-9c941d4682b7" xlink:href="ck1437958-20260630.xsd#ck1437958_ConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7824-a9b8-374c8f84fe2b" xlink:to="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-7f95-993c-9c941d4682b7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7189-966a-38340f38cf4b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableTroubledDebtRestructuringsTable_019edbd5-e61b-7fb0-9ea9-0226902e4813" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7189-966a-38340f38cf4b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7189-966a-38340f38cf4b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7189-966a-38340f38cf4b" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7189-966a-38340f38cf4b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-78fd-8d90-c280717c548f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7189-966a-38340f38cf4b" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-78fd-8d90-c280717c548f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXCapitalCallLinesMember_019edbd5-e61b-7c54-bf05-58d903faa8f2" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXCapitalCallLinesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-78fd-8d90-c280717c548f" xlink:to="loc_ck1437958_CCBXCapitalCallLinesMember_019edbd5-e61b-7c54-bf05-58d903faa8f2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXOtherCommercialAndIndustrialLoansMember_019edbd5-e61b-70cc-99f6-577c5b847bec" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXOtherCommercialAndIndustrialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-78fd-8d90-c280717c548f" xlink:to="loc_ck1437958_CCBXOtherCommercialAndIndustrialLoansMember_019edbd5-e61b-70cc-99f6-577c5b847bec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-7883-bc94-0ddc24f2bbaa" xlink:href="ck1437958-20260630.xsd#ck1437958_ResidentialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-78fd-8d90-c280717c548f" xlink:to="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-7883-bc94-0ddc24f2bbaa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditCardMember_019edbd5-e61b-7547-abcb-12c16a9e777a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditCardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-78fd-8d90-c280717c548f" xlink:to="loc_us-gaap_CreditCardMember_019edbd5-e61b-7547-abcb-12c16a9e777a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_OtherConsumerAndOtherLoansMember_019edbd5-e61b-7e7b-93f4-17e2d2d16279" xlink:href="ck1437958-20260630.xsd#ck1437958_OtherConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-78fd-8d90-c280717c548f" xlink:to="loc_ck1437958_OtherConsumerAndOtherLoansMember_019edbd5-e61b-7e7b-93f4-17e2d2d16279" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019edbd5-e61b-7ea6-a295-5211270589fb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablesPeriodPastDueAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableTroubledDebtRestructuringsTable_019edbd5-e61b-7fb0-9ea9-0226902e4813" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019edbd5-e61b-7ea6-a295-5211270589fb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7ea6-a295-5211270589fb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablesPeriodPastDueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019edbd5-e61b-7ea6-a295-5211270589fb" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7ea6-a295-5211270589fb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7b51-8a6c-2ea00c888afa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablesPeriodPastDueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueAxis_019edbd5-e61b-7ea6-a295-5211270589fb" xlink:to="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7b51-8a6c-2ea00c888afa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_FinancialAsset30To89DaysPastDueMember_019edbd5-e61b-7272-96f2-dcf93c6be414" xlink:href="ck1437958-20260630.xsd#ck1437958_FinancialAsset30To89DaysPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7b51-8a6c-2ea00c888afa" xlink:to="loc_ck1437958_FinancialAsset30To89DaysPastDueMember_019edbd5-e61b-7272-96f2-dcf93c6be414" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember_019edbd5-e61b-74df-b811-162f8d7675ce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7b51-8a6c-2ea00c888afa" xlink:to="loc_us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember_019edbd5-e61b-74df-b811-162f8d7675ce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialAssetPastDueMember_019edbd5-e61b-74ca-b12c-2b2645761d12" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialAssetPastDueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablesPeriodPastDueDomain_019edbd5-e61b-7b51-8a6c-2ea00c888afa" xlink:to="loc_us-gaap_FinancialAssetPastDueMember_019edbd5-e61b-74ca-b12c-2b2645761d12" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestModifiedPeriod_019edbd5-e61b-7407-80ea-bc8dac54961c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestModifiedPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableModificationsLineItems_019edbd5-e61b-7e0c-bdfa-010335d9a23a" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestModifiedPeriod_019edbd5-e61b-7407-80ea-bc8dac54961c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesExcludingAccruedInterestPercent_019edbd5-e61b-7343-b9cc-fe5d31f62c92" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesExcludingAccruedInterestPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableModificationsLineItems_019edbd5-e61b-7e0c-bdfa-010335d9a23a" xlink:to="loc_us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesExcludingAccruedInterestPercent_019edbd5-e61b-7343-b9cc-fe5d31f62c92" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestModifiedPast12Months_019edbd5-e61b-77b4-aa06-7db94e1e8d04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestModifiedPast12Months"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableModificationsLineItems_019edbd5-e61b-7e0c-bdfa-010335d9a23a" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestModifiedPast12Months_019edbd5-e61b-77b4-aa06-7db94e1e8d04" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableModifiedWeightedAverageInterestRateDecreaseFromModification_019edbd5-e61b-7b9c-8390-4c87fe1834bb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableModifiedWeightedAverageInterestRateDecreaseFromModification"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableModificationsLineItems_019edbd5-e61b-7e0c-bdfa-010335d9a23a" xlink:to="loc_us-gaap_FinancingReceivableModifiedWeightedAverageInterestRateDecreaseFromModification_019edbd5-e61b-7b9c-8390-4c87fe1834bb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableModifiedWeightedAverageTermIncreaseFromModification_019edbd5-e61b-7f3e-95a9-0a3890043751" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableModifiedWeightedAverageTermIncreaseFromModification"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableModificationsLineItems_019edbd5-e61b-7e0c-bdfa-010335d9a23a" xlink:to="loc_us-gaap_FinancingReceivableModifiedWeightedAverageTermIncreaseFromModification_019edbd5-e61b-7f3e-95a9-0a3890043751" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestModifiedSubsequentDefault_019edbd5-e61b-7745-9b11-205480d211aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestModifiedSubsequentDefault"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableModificationsLineItems_019edbd5-e61b-7e0c-bdfa-010335d9a23a" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestModifiedSubsequentDefault_019edbd5-e61b-7745-9b11-205480d211aa" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansAndLeasesReceivableImpairedCommitmentToLend_019edbd5-e61b-7114-b024-3f21a59f6b1d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansAndLeasesReceivableImpairedCommitmentToLend"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableModificationsLineItems_019edbd5-e61b-7e0c-bdfa-010335d9a23a" xlink:to="loc_us-gaap_LoansAndLeasesReceivableImpairedCommitmentToLend_019edbd5-e61b-7114-b024-3f21a59f6b1d" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesSummaryofRiskCategoryofLoansbyClassofLoansDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#LoansandAllowanceforCreditLossesSummaryofRiskCategoryofLoansbyClassofLoansDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesSummaryofRiskCategoryofLoansbyClassofLoansDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivableRecordedInvestmentCreditQualityIndicatorTable_019edbd5-e61b-77c2-8ff6-ff561bcacc7c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfFinancingReceivableRecordedInvestmentCreditQualityIndicatorTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivableRecordedInvestmentCreditQualityIndicatorTable_019edbd5-e61b-77c2-8ff6-ff561bcacc7c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7dcb-8917-17fa68246dc6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableRecordedInvestmentCreditQualityIndicatorTable_019edbd5-e61b-77c2-8ff6-ff561bcacc7c" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7dcb-8917-17fa68246dc6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7dcb-8917-17fa68246dc6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7dcb-8917-17fa68246dc6" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7dcb-8917-17fa68246dc6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7cae-a2e9-7c9304a5b1f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7dcb-8917-17fa68246dc6" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7cae-a2e9-7c9304a5b1f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_TotalCommunityBankLoansPortfolioSegmentMember_019edbd5-e61b-77dd-9f9f-3c3b4c8b238d" xlink:href="ck1437958-20260630.xsd#ck1437958_TotalCommunityBankLoansPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7cae-a2e9-7c9304a5b1f0" xlink:to="loc_ck1437958_TotalCommunityBankLoansPortfolioSegmentMember_019edbd5-e61b-77dd-9f9f-3c3b4c8b238d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019edbd5-e61b-7ad9-84ea-9d9748f5f152" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ck1437958_TotalCommunityBankLoansPortfolioSegmentMember_019edbd5-e61b-77dd-9f9f-3c3b4c8b238d" xlink:to="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019edbd5-e61b-7ad9-84ea-9d9748f5f152" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-75e5-bd93-24aa51827f7f" xlink:href="ck1437958-20260630.xsd#ck1437958_RealEstatePortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ck1437958_TotalCommunityBankLoansPortfolioSegmentMember_019edbd5-e61b-77dd-9f9f-3c3b4c8b238d" xlink:to="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-75e5-bd93-24aa51827f7f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-78da-9be8-b57247ff8562" xlink:href="ck1437958-20260630.xsd#ck1437958_ConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ck1437958_TotalCommunityBankLoansPortfolioSegmentMember_019edbd5-e61b-77dd-9f9f-3c3b4c8b238d" xlink:to="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-78da-9be8-b57247ff8562" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7339-a034-c362e36dea52" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableRecordedInvestmentCreditQualityIndicatorTable_019edbd5-e61b-77c2-8ff6-ff561bcacc7c" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7339-a034-c362e36dea52" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7339-a034-c362e36dea52_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7339-a034-c362e36dea52" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7339-a034-c362e36dea52_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-777c-afc8-7fb16ec3ff06" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7339-a034-c362e36dea52" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-777c-afc8-7fb16ec3ff06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember_019edbd5-e61b-7566-8984-797fbdf29d8b" xlink:href="ck1437958-20260630.xsd#ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-777c-afc8-7fb16ec3ff06" xlink:to="loc_ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember_019edbd5-e61b-7566-8984-797fbdf29d8b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-717c-8271-5016bf31312b" xlink:href="ck1437958-20260630.xsd#ck1437958_ResidentialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-777c-afc8-7fb16ec3ff06" xlink:to="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-717c-8271-5016bf31312b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialRealEstateLoansMember_019edbd5-e61b-77c5-88c1-4ea12d69c0e9" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-777c-afc8-7fb16ec3ff06" xlink:to="loc_ck1437958_CommercialRealEstateLoansMember_019edbd5-e61b-77c5-88c1-4ea12d69c0e9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InternalCreditAssessmentAxis_019edbd5-e61b-7062-8ed3-651270335f96" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InternalCreditAssessmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableRecordedInvestmentCreditQualityIndicatorTable_019edbd5-e61b-77c2-8ff6-ff561bcacc7c" xlink:to="loc_us-gaap_InternalCreditAssessmentAxis_019edbd5-e61b-7062-8ed3-651270335f96" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InternalCreditAssessmentDomain_019edbd5-e61b-7062-8ed3-651270335f96_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InternalCreditAssessmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_InternalCreditAssessmentAxis_019edbd5-e61b-7062-8ed3-651270335f96" xlink:to="loc_us-gaap_InternalCreditAssessmentDomain_019edbd5-e61b-7062-8ed3-651270335f96_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InternalCreditAssessmentDomain_019edbd5-e61b-72d0-abe5-634d9d3fe63a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InternalCreditAssessmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_InternalCreditAssessmentAxis_019edbd5-e61b-7062-8ed3-651270335f96" xlink:to="loc_us-gaap_InternalCreditAssessmentDomain_019edbd5-e61b-72d0-abe5-634d9d3fe63a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PassMember_019edbd5-e61b-7baf-9a96-c13ec5a8dceb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PassMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InternalCreditAssessmentDomain_019edbd5-e61b-72d0-abe5-634d9d3fe63a" xlink:to="loc_us-gaap_PassMember_019edbd5-e61b-7baf-9a96-c13ec5a8dceb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SpecialMentionMember_019edbd5-e61b-7e99-92b9-c1bb23b86923" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SpecialMentionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InternalCreditAssessmentDomain_019edbd5-e61b-72d0-abe5-634d9d3fe63a" xlink:to="loc_us-gaap_SpecialMentionMember_019edbd5-e61b-7e99-92b9-c1bb23b86923" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubstandardMember_019edbd5-e61b-74f7-a03b-0089ef11388f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubstandardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InternalCreditAssessmentDomain_019edbd5-e61b-72d0-abe5-634d9d3fe63a" xlink:to="loc_us-gaap_SubstandardMember_019edbd5-e61b-74f7-a03b-0089ef11388f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DoubtfulMember_019edbd5-e61b-72ed-bffd-ea6004635c3d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DoubtfulMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InternalCreditAssessmentDomain_019edbd5-e61b-72d0-abe5-634d9d3fe63a" xlink:to="loc_us-gaap_DoubtfulMember_019edbd5-e61b-72ed-bffd-ea6004635c3d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYear_019edbd5-e61b-721d-93ad-2cb41c545894" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYear_019edbd5-e61b-721d-93ad-2cb41c545894" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYearWriteoff_019edbd5-e61b-70df-899c-30ec5d8dbcfc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYearWriteoff_019edbd5-e61b-70df-899c-30ec5d8dbcfc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYear_019edbd5-e61b-7230-ae07-a913cc489e8d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYear_019edbd5-e61b-7230-ae07-a913cc489e8d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-7fb2-b80d-e312cacbcd1f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-7fb2-b80d-e312cacbcd1f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYear_019edbd5-e61b-7d1b-80b2-049687d0104b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYear_019edbd5-e61b-7d1b-80b2-049687d0104b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-735f-8fa3-84a03879c973" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-735f-8fa3-84a03879c973" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYear_019edbd5-e61b-7385-90f4-a8070259c486" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYear_019edbd5-e61b-7385-90f4-a8070259c486" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-7fde-9d59-a7779e47e9af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-7fde-9d59-a7779e47e9af" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYear_019edbd5-e61b-7a95-b12d-4e8efeef367a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYear_019edbd5-e61b-7a95-b12d-4e8efeef367a" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-7261-bdd1-85bbfdd1b547" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-7261-bdd1-85bbfdd1b547" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYear_019edbd5-e61b-7632-83d4-c3284b292c98" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYear_019edbd5-e61b-7632-83d4-c3284b292c98" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-7688-bccc-00514729df3a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-7688-bccc-00514729df3a" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestRevolving_019edbd5-e61b-7366-b1b9-d93f5a76c322" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestRevolving"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestRevolving_019edbd5-e61b-7366-b1b9-d93f5a76c322" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingWriteoff_019edbd5-e61b-7cab-8ddf-334723bb4fce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingWriteoff_019edbd5-e61b-7cab-8ddf-334723bb4fce" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingConvertedToTermLoan_019edbd5-e61b-7ec6-984b-9ff80e4ca037" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingConvertedToTermLoan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingConvertedToTermLoan_019edbd5-e61b-7ec6-984b-9ff80e4ca037" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_FinancingReceivableExcludingAccruedInterestRevolvingAndConvertedToTermWriteoff_019edbd5-e61b-7dd4-b5e0-73503b3a6ded" xlink:href="ck1437958-20260630.xsd#ck1437958_FinancingReceivableExcludingAccruedInterestRevolvingAndConvertedToTermWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_ck1437958_FinancingReceivableExcludingAccruedInterestRevolvingAndConvertedToTermWriteoff_019edbd5-e61b-7dd4-b5e0-73503b3a6ded" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-774a-9dec-4f1a11d27820" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-774a-9dec-4f1a11d27820" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossWriteoff_019edbd5-e61b-7dfe-ac8f-1f502ea0699d" xlink:href="ck1437958-20260630.xsd#ck1437958_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7d7c-a9f8-7c1c71dc1eef" xlink:to="loc_ck1437958_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossWriteoff_019edbd5-e61b-7dfe-ac8f-1f502ea0699d" xlink:type="arc" order="17"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesSummaryofAmortizedCostofCCBXLoansDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#LoansandAllowanceforCreditLossesSummaryofAmortizedCostofCCBXLoansDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesSummaryofAmortizedCostofCCBXLoansDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivableRecordedInvestmentCreditQualityIndicatorTable_019edbd5-e61b-7eab-a64b-b24063334f4f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfFinancingReceivableRecordedInvestmentCreditQualityIndicatorTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivableRecordedInvestmentCreditQualityIndicatorTable_019edbd5-e61b-7eab-a64b-b24063334f4f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentPerformanceStatusAxis_019edbd5-e61b-7a7a-87b3-0a85ac37da73" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentPerformanceStatusAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableRecordedInvestmentCreditQualityIndicatorTable_019edbd5-e61b-7eab-a64b-b24063334f4f" xlink:to="loc_us-gaap_FinancialInstrumentPerformanceStatusAxis_019edbd5-e61b-7a7a-87b3-0a85ac37da73" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentPerformanceStatusDomain_019edbd5-e61b-7a7a-87b3-0a85ac37da73_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentPerformanceStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentPerformanceStatusAxis_019edbd5-e61b-7a7a-87b3-0a85ac37da73" xlink:to="loc_us-gaap_FinancialInstrumentPerformanceStatusDomain_019edbd5-e61b-7a7a-87b3-0a85ac37da73_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentPerformanceStatusDomain_019edbd5-e61b-7373-8caa-08ca66d7b06e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentPerformanceStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentPerformanceStatusAxis_019edbd5-e61b-7a7a-87b3-0a85ac37da73" xlink:to="loc_us-gaap_FinancialInstrumentPerformanceStatusDomain_019edbd5-e61b-7373-8caa-08ca66d7b06e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformingFinancingReceivableMember_019edbd5-e61b-7bde-9703-a3534c5ab8ef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformingFinancingReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentPerformanceStatusDomain_019edbd5-e61b-7373-8caa-08ca66d7b06e" xlink:to="loc_us-gaap_PerformingFinancingReceivableMember_019edbd5-e61b-7bde-9703-a3534c5ab8ef" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonperformingFinancingReceivableMember_019edbd5-e61b-74cf-8844-67d359f05ae1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonperformingFinancingReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentPerformanceStatusDomain_019edbd5-e61b-7373-8caa-08ca66d7b06e" xlink:to="loc_us-gaap_NonperformingFinancingReceivableMember_019edbd5-e61b-74cf-8844-67d359f05ae1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7b4d-8008-be32cd3df429" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableRecordedInvestmentCreditQualityIndicatorTable_019edbd5-e61b-7eab-a64b-b24063334f4f" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7b4d-8008-be32cd3df429" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7b4d-8008-be32cd3df429_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7b4d-8008-be32cd3df429" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7b4d-8008-be32cd3df429_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7899-a79e-56cfd4292e97" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-7b4d-8008-be32cd3df429" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7899-a79e-56cfd4292e97" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_TotalCCBXLoansMember_019edbd5-e61b-7bdc-9374-30027601296c" xlink:href="ck1437958-20260630.xsd#ck1437958_TotalCCBXLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7899-a79e-56cfd4292e97" xlink:to="loc_ck1437958_TotalCCBXLoansMember_019edbd5-e61b-7bdc-9374-30027601296c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019edbd5-e61b-7433-b29e-b44ef123dcb0" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ck1437958_TotalCCBXLoansMember_019edbd5-e61b-7bdc-9374-30027601296c" xlink:to="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019edbd5-e61b-7433-b29e-b44ef123dcb0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-75df-9ef4-f4053e40ca52" xlink:href="ck1437958-20260630.xsd#ck1437958_RealEstatePortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ck1437958_TotalCCBXLoansMember_019edbd5-e61b-7bdc-9374-30027601296c" xlink:to="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-75df-9ef4-f4053e40ca52" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-7346-96a1-98ebfcadca3c" xlink:href="ck1437958-20260630.xsd#ck1437958_ConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ck1437958_TotalCCBXLoansMember_019edbd5-e61b-7bdc-9374-30027601296c" xlink:to="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-7346-96a1-98ebfcadca3c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7f8a-9c0d-a4f482006397" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableRecordedInvestmentCreditQualityIndicatorTable_019edbd5-e61b-7eab-a64b-b24063334f4f" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7f8a-9c0d-a4f482006397" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7f8a-9c0d-a4f482006397_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7f8a-9c0d-a4f482006397" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7f8a-9c0d-a4f482006397_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-71f1-983b-1a04c1d7f06b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7f8a-9c0d-a4f482006397" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-71f1-983b-1a04c1d7f06b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXCapitalCallLinesMember_019edbd5-e61b-7c3f-ab41-84b18a59c18c" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXCapitalCallLinesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-71f1-983b-1a04c1d7f06b" xlink:to="loc_ck1437958_CCBXCapitalCallLinesMember_019edbd5-e61b-7c3f-ab41-84b18a59c18c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXOtherCommercialAndIndustrialLoansMember_019edbd5-e61b-709b-9f3a-a8e7c2a3ef57" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXOtherCommercialAndIndustrialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-71f1-983b-1a04c1d7f06b" xlink:to="loc_ck1437958_CCBXOtherCommercialAndIndustrialLoansMember_019edbd5-e61b-709b-9f3a-a8e7c2a3ef57" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-73d8-a2f5-8a5923608fd6" xlink:href="ck1437958-20260630.xsd#ck1437958_ResidentialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-71f1-983b-1a04c1d7f06b" xlink:to="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-73d8-a2f5-8a5923608fd6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditCardMember_019edbd5-e61b-7a1b-a795-33192cff8a48" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditCardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-71f1-983b-1a04c1d7f06b" xlink:to="loc_us-gaap_CreditCardMember_019edbd5-e61b-7a1b-a795-33192cff8a48" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_OtherConsumerAndOtherLoansMember_019edbd5-e61b-7b2c-8649-a08d71896b42" xlink:href="ck1437958-20260630.xsd#ck1437958_OtherConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-71f1-983b-1a04c1d7f06b" xlink:to="loc_ck1437958_OtherConsumerAndOtherLoansMember_019edbd5-e61b-7b2c-8649-a08d71896b42" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019edbd5-e61b-7ef1-93e9-207a970c39b8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableRecordedInvestmentCreditQualityIndicatorTable_019edbd5-e61b-7eab-a64b-b24063334f4f" xlink:to="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019edbd5-e61b-7ef1-93e9-207a970c39b8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-7ef1-93e9-207a970c39b8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019edbd5-e61b-7ef1-93e9-207a970c39b8" xlink:to="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-7ef1-93e9-207a970c39b8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-7223-be73-7e44096291ee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019edbd5-e61b-7ef1-93e9-207a970c39b8" xlink:to="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-7223-be73-7e44096291ee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXLoansMember_019edbd5-e61b-76e4-a374-c14ad31ba1c2" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-7223-be73-7e44096291ee" xlink:to="loc_ck1437958_CCBXLoansMember_019edbd5-e61b-76e4-a374-c14ad31ba1c2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYear_019edbd5-e61b-7455-ac16-352f30e04d71" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYear_019edbd5-e61b-7455-ac16-352f30e04d71" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYearWriteoff_019edbd5-e61b-7dae-83c1-11d19e30632e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearOneOriginatedCurrentFiscalYearWriteoff_019edbd5-e61b-7dae-83c1-11d19e30632e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYear_019edbd5-e61b-7e74-afa6-10c49f1efe01" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYear_019edbd5-e61b-7e74-afa6-10c49f1efe01" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-746c-9641-f9e68c532f6a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-746c-9641-f9e68c532f6a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYear_019edbd5-e61b-7c95-9b74-abcc004a322f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYear_019edbd5-e61b-7c95-9b74-abcc004a322f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-7879-989d-6a24a132b1f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearThreeOriginatedTwoYearsBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-7879-989d-6a24a132b1f2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYear_019edbd5-e61b-7dd2-98c6-8757497a6d6e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYear_019edbd5-e61b-7dd2-98c6-8757497a6d6e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-77ef-9c6e-87f7b19287b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearFourOriginatedThreeYearsBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-77ef-9c6e-87f7b19287b3" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYear_019edbd5-e61b-7faa-98c1-c79c17e46f25" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYear_019edbd5-e61b-7faa-98c1-c79c17e46f25" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-7657-8c5e-43c9c54665db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestYearFiveOriginatedFourYearsBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-7657-8c5e-43c9c54665db" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYear_019edbd5-e61b-7482-9f26-e57d2a43b001" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYear_019edbd5-e61b-7482-9f26-e57d2a43b001" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-77f4-8c29-afe2d5c4960d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestOriginatedMoreThanFiveYearsBeforeCurrentFiscalYearWriteoff_019edbd5-e61b-77f4-8c29-afe2d5c4960d" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestRevolving_019edbd5-e61b-72c9-9597-49b83706c458" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestRevolving"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestRevolving_019edbd5-e61b-72c9-9597-49b83706c458" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingWriteoff_019edbd5-e61b-7bbc-b2a4-cd167f035301" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingWriteoff_019edbd5-e61b-7bbc-b2a4-cd167f035301" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingConvertedToTermLoan_019edbd5-e61b-7d8f-ad16-836a1e43e1b0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingConvertedToTermLoan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestRevolvingConvertedToTermLoan_019edbd5-e61b-7d8f-ad16-836a1e43e1b0" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_FinancingReceivableExcludingAccruedInterestRevolvingAndConvertedToTermWriteoff_019edbd5-e61b-7fe9-b04f-1ff53228d4e4" xlink:href="ck1437958-20260630.xsd#ck1437958_FinancingReceivableExcludingAccruedInterestRevolvingAndConvertedToTermWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_ck1437958_FinancingReceivableExcludingAccruedInterestRevolvingAndConvertedToTermWriteoff_019edbd5-e61b-7fe9-b04f-1ff53228d4e4" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-7572-af96-5be85241dfd3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-7572-af96-5be85241dfd3" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossWriteoff_019edbd5-e61b-7321-a279-689c96d1d6c3" xlink:href="ck1437958-20260630.xsd#ck1437958_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossWriteoff"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentLineItems_019edbd5-e61b-7b62-83bf-864c0287cf0c" xlink:to="loc_ck1437958_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossWriteoff_019edbd5-e61b-7321-a279-689c96d1d6c3" xlink:type="arc" order="17"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesSummaryofLoansbyCreditQualityRiskRatingDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#LoansandAllowanceforCreditLossesSummaryofLoansbyCreditQualityRiskRatingDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesSummaryofLoansbyCreditQualityRiskRatingDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansAndLeasesReceivableDisclosureLineItems_019edbd5-e61b-752c-b52b-ac7b2284f142" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansAndLeasesReceivableDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansAndLeasesReceivableDisclosureTable_019edbd5-e61b-7191-8808-16c311453953" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansAndLeasesReceivableDisclosureTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LoansAndLeasesReceivableDisclosureLineItems_019edbd5-e61b-752c-b52b-ac7b2284f142" xlink:to="loc_us-gaap_LoansAndLeasesReceivableDisclosureTable_019edbd5-e61b-7191-8808-16c311453953" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InternalCreditAssessmentAxis_019edbd5-e61b-7849-8454-b3d8a5838d38" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InternalCreditAssessmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LoansAndLeasesReceivableDisclosureTable_019edbd5-e61b-7191-8808-16c311453953" xlink:to="loc_us-gaap_InternalCreditAssessmentAxis_019edbd5-e61b-7849-8454-b3d8a5838d38" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InternalCreditAssessmentDomain_019edbd5-e61b-7849-8454-b3d8a5838d38_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InternalCreditAssessmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_InternalCreditAssessmentAxis_019edbd5-e61b-7849-8454-b3d8a5838d38" xlink:to="loc_us-gaap_InternalCreditAssessmentDomain_019edbd5-e61b-7849-8454-b3d8a5838d38_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InternalCreditAssessmentDomain_019edbd5-e61b-79c1-ac6b-bfbe83c99ec2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InternalCreditAssessmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_InternalCreditAssessmentAxis_019edbd5-e61b-7849-8454-b3d8a5838d38" xlink:to="loc_us-gaap_InternalCreditAssessmentDomain_019edbd5-e61b-79c1-ac6b-bfbe83c99ec2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PassMember_019edbd5-e61b-75ad-ace8-bb1cb5fc9db2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PassMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InternalCreditAssessmentDomain_019edbd5-e61b-79c1-ac6b-bfbe83c99ec2" xlink:to="loc_us-gaap_PassMember_019edbd5-e61b-75ad-ace8-bb1cb5fc9db2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubstandardMember_019edbd5-e61b-7eed-82f9-54301d0103e5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubstandardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InternalCreditAssessmentDomain_019edbd5-e61b-79c1-ac6b-bfbe83c99ec2" xlink:to="loc_us-gaap_SubstandardMember_019edbd5-e61b-7eed-82f9-54301d0103e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DoubtfulMember_019edbd5-e61b-795b-98df-6546591da56b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DoubtfulMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InternalCreditAssessmentDomain_019edbd5-e61b-79c1-ac6b-bfbe83c99ec2" xlink:to="loc_us-gaap_DoubtfulMember_019edbd5-e61b-795b-98df-6546591da56b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SpecialMentionMember_019edbd5-e61b-721c-ba7c-2fe782c302fb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SpecialMentionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InternalCreditAssessmentDomain_019edbd5-e61b-79c1-ac6b-bfbe83c99ec2" xlink:to="loc_us-gaap_SpecialMentionMember_019edbd5-e61b-721c-ba7c-2fe782c302fb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-75db-81d0-a2dc8e30a341" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LoansAndLeasesReceivableDisclosureTable_019edbd5-e61b-7191-8808-16c311453953" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-75db-81d0-a2dc8e30a341" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-75db-81d0-a2dc8e30a341_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-75db-81d0-a2dc8e30a341" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-75db-81d0-a2dc8e30a341_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7b9d-97c6-c30be344960a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-75db-81d0-a2dc8e30a341" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7b9d-97c6-c30be344960a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019edbd5-e61b-7116-87e0-e69e001402a2" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7b9d-97c6-c30be344960a" xlink:to="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019edbd5-e61b-7116-87e0-e69e001402a2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember_019edbd5-e61b-721f-b2c8-2e7673301ebc" xlink:href="ck1437958-20260630.xsd#ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7b9d-97c6-c30be344960a" xlink:to="loc_ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember_019edbd5-e61b-721f-b2c8-2e7673301ebc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-7777-9fcc-a2c8879de12f" xlink:href="ck1437958-20260630.xsd#ck1437958_RealEstatePortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7b9d-97c6-c30be344960a" xlink:to="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-7777-9fcc-a2c8879de12f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-7703-ae4a-88abe1d760cd" xlink:href="ck1437958-20260630.xsd#ck1437958_ConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7b9d-97c6-c30be344960a" xlink:to="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-7703-ae4a-88abe1d760cd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-79b4-8818-b6c2f9308192" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LoansAndLeasesReceivableDisclosureTable_019edbd5-e61b-7191-8808-16c311453953" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-79b4-8818-b6c2f9308192" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-79b4-8818-b6c2f9308192_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-79b4-8818-b6c2f9308192" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-79b4-8818-b6c2f9308192_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7fe4-b45f-6e96badda212" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-79b4-8818-b6c2f9308192" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7fe4-b45f-6e96badda212" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-74a6-88d5-6a1f93862b0a" xlink:href="ck1437958-20260630.xsd#ck1437958_ResidentialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7fe4-b45f-6e96badda212" xlink:to="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-74a6-88d5-6a1f93862b0a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialRealEstateLoansMember_019edbd5-e61b-7408-b2fd-0e4881614d39" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7fe4-b45f-6e96badda212" xlink:to="loc_ck1437958_CommercialRealEstateLoansMember_019edbd5-e61b-7408-b2fd-0e4881614d39" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess_019edbd5-e61b-721f-b765-a00b4af8d9de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoansAndLeasesReceivableDisclosureLineItems_019edbd5-e61b-752c-b52b-ac7b2284f142" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess_019edbd5-e61b-721f-b765-a00b4af8d9de" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount_019edbd5-e61b-714c-bfeb-1aea400a44a7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoansAndLeasesReceivableDisclosureLineItems_019edbd5-e61b-752c-b52b-ac7b2284f142" xlink:to="loc_us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount_019edbd5-e61b-714c-bfeb-1aea400a44a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-7530-8254-f16deed544f5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoansAndLeasesReceivableDisclosureLineItems_019edbd5-e61b-752c-b52b-ac7b2284f142" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e61b-7530-8254-f16deed544f5" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesSummaryofAllocationofAllowanceforLoanLossaswellasActivityinAllowanceforLoanLossAttributedtoVariousSegmentsinLoanDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#LoansandAllowanceforCreditLossesSummaryofAllocationofAllowanceforLoanLossaswellasActivityinAllowanceforLoanLossAttributedtoVariousSegmentsinLoanDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesSummaryofAllocationofAllowanceforLoanLossaswellasActivityinAllowanceforLoanLossAttributedtoVariousSegmentsinLoanDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansAndLeasesReceivableDisclosureLineItems_019edbd5-e61b-7ac3-a6e4-f68cbd126aed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansAndLeasesReceivableDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansAndLeasesReceivableDisclosureTable_019edbd5-e61b-7485-8ecb-4e36fd89b377" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansAndLeasesReceivableDisclosureTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LoansAndLeasesReceivableDisclosureLineItems_019edbd5-e61b-7ac3-a6e4-f68cbd126aed" xlink:to="loc_us-gaap_LoansAndLeasesReceivableDisclosureTable_019edbd5-e61b-7485-8ecb-4e36fd89b377" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-779c-8841-d9bf8a707458" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LoansAndLeasesReceivableDisclosureTable_019edbd5-e61b-7485-8ecb-4e36fd89b377" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-779c-8841-d9bf8a707458" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-779c-8841-d9bf8a707458_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-779c-8841-d9bf8a707458" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-779c-8841-d9bf8a707458_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7b93-94f0-01d4994523d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-779c-8841-d9bf8a707458" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7b93-94f0-01d4994523d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019edbd5-e61b-7f81-b80b-59fef9e67556" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7b93-94f0-01d4994523d8" xlink:to="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019edbd5-e61b-7f81-b80b-59fef9e67556" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember_019edbd5-e61b-7fad-a0fc-3900f64cc04e" xlink:href="ck1437958-20260630.xsd#ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7b93-94f0-01d4994523d8" xlink:to="loc_ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember_019edbd5-e61b-7fad-a0fc-3900f64cc04e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-7b25-a0ce-45cb2acfe5d9" xlink:href="ck1437958-20260630.xsd#ck1437958_RealEstatePortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7b93-94f0-01d4994523d8" xlink:to="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-7b25-a0ce-45cb2acfe5d9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-7d2d-a689-8c55a1680060" xlink:href="ck1437958-20260630.xsd#ck1437958_ConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-7b93-94f0-01d4994523d8" xlink:to="loc_ck1437958_ConsumerAndOtherLoansMember_019edbd5-e61b-7d2d-a689-8c55a1680060" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-78f7-b993-632f5aa632b2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LoansAndLeasesReceivableDisclosureTable_019edbd5-e61b-7485-8ecb-4e36fd89b377" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-78f7-b993-632f5aa632b2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-78f7-b993-632f5aa632b2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-78f7-b993-632f5aa632b2" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-78f7-b993-632f5aa632b2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-75d0-9d58-8df0e141f621" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-78f7-b993-632f5aa632b2" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-75d0-9d58-8df0e141f621" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-717c-b53f-18295fc4e6bd" xlink:href="ck1437958-20260630.xsd#ck1437958_ResidentialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-75d0-9d58-8df0e141f621" xlink:to="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-717c-b53f-18295fc4e6bd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialRealEstateLoansMember_019edbd5-e61b-7b56-b8ad-6f056ed3e8a1" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-75d0-9d58-8df0e141f621" xlink:to="loc_ck1437958_CommercialRealEstateLoansMember_019edbd5-e61b-7b56-b8ad-6f056ed3e8a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRollForward_019edbd5-e61b-7f5d-84e5-037e754c5d5f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoansAndLeasesReceivableDisclosureLineItems_019edbd5-e61b-7ac3-a6e4-f68cbd126aed" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRollForward_019edbd5-e61b-7f5d-84e5-037e754c5d5f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest_019edbd5-e61b-7d91-adfd-25ae05a370d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRollForward_019edbd5-e61b-7f5d-84e5-037e754c5d5f" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest_019edbd5-e61b-7d91-adfd-25ae05a370d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProvisionForLoanLossesExpensed_019edbd5-e61b-7b2b-9ffb-b9f81429093c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProvisionForLoanLossesExpensed"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRollForward_019edbd5-e61b-7f5d-84e5-037e754c5d5f" xlink:to="loc_us-gaap_ProvisionForLoanLossesExpensed_019edbd5-e61b-7b2b-9ffb-b9f81429093c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_FinancingReceivableAllowanceForCreditLossAfterExpenseReversal_019edbd5-e61b-750a-a8ec-5fd613b7082d" xlink:href="ck1437958-20260630.xsd#ck1437958_FinancingReceivableAllowanceForCreditLossAfterExpenseReversal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRollForward_019edbd5-e61b-7f5d-84e5-037e754c5d5f" xlink:to="loc_ck1437958_FinancingReceivableAllowanceForCreditLossAfterExpenseReversal_019edbd5-e61b-750a-a8ec-5fd613b7082d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs_019edbd5-e61b-7017-bd82-17569a0fd04d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRollForward_019edbd5-e61b-7f5d-84e5-037e754c5d5f" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs_019edbd5-e61b-7017-bd82-17569a0fd04d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery_019edbd5-e61b-7fbf-bb73-0ed773a53121" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRollForward_019edbd5-e61b-7f5d-84e5-037e754c5d5f" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery_019edbd5-e61b-7fbf-bb73-0ed773a53121" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossWriteoffAfterRecovery_019edbd5-e61b-7411-80f1-51fd94dd8d1e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossWriteoffAfterRecovery"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRollForward_019edbd5-e61b-7f5d-84e5-037e754c5d5f" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossWriteoffAfterRecovery_019edbd5-e61b-7411-80f1-51fd94dd8d1e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest_019edbd5-e61b-7199-831b-60cc3d53e871" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment_019edbd5-e61b-781d-9743-c8d393de6b71" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoansAndLeasesReceivableDisclosureLineItems_019edbd5-e61b-7ac3-a6e4-f68cbd126aed" xlink:to="loc_us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment_019edbd5-e61b-781d-9743-c8d393de6b71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess_019edbd5-e61b-7abc-b340-31d80757c2bf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoansAndLeasesReceivableDisclosureLineItems_019edbd5-e61b-7ac3-a6e4-f68cbd126aed" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossFeeAndLoanInProcess_019edbd5-e61b-7abc-b340-31d80757c2bf" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesSummaryofCollateralDependentLoansDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#LoansandAllowanceforCreditLossesSummaryofCollateralDependentLoansDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/LoansandAllowanceforCreditLossesSummaryofCollateralDependentLoansDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-7744-9143-f7523be8889a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-73bd-958b-36a6a8f1a771" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-7744-9143-f7523be8889a" xlink:to="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-73bd-958b-36a6a8f1a771" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-76b5-ac7e-16df4b574958" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-73bd-958b-36a6a8f1a771" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-76b5-ac7e-16df4b574958" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-76b5-ac7e-16df4b574958_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-76b5-ac7e-16df4b574958" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-76b5-ac7e-16df4b574958_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-79cf-8c0b-d95f84d5f83a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivablePortfolioSegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentAxis_019edbd5-e61b-76b5-ac7e-16df4b574958" xlink:to="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-79cf-8c0b-d95f84d5f83a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019edbd5-e61b-716d-904b-1b89808126d4" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-79cf-8c0b-d95f84d5f83a" xlink:to="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019edbd5-e61b-716d-904b-1b89808126d4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-78d9-bf13-19ea302a69ae" xlink:href="ck1437958-20260630.xsd#ck1437958_RealEstatePortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-79cf-8c0b-d95f84d5f83a" xlink:to="loc_ck1437958_RealEstatePortfolioSegmentMember_019edbd5-e61b-78d9-bf13-19ea302a69ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember_019edbd5-e61b-79e1-82e5-55760969b6c0" xlink:href="ck1437958-20260630.xsd#ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivablePortfolioSegmentDomain_019edbd5-e61b-79cf-8c0b-d95f84d5f83a" xlink:to="loc_ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember_019edbd5-e61b-79e1-82e5-55760969b6c0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7cef-a2ca-0ab13aa222d6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-73bd-958b-36a6a8f1a771" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7cef-a2ca-0ab13aa222d6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7cef-a2ca-0ab13aa222d6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7cef-a2ca-0ab13aa222d6" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7cef-a2ca-0ab13aa222d6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7ce1-9d68-7dd84b46ddec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019edbd5-e61b-7cef-a2ca-0ab13aa222d6" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7ce1-9d68-7dd84b46ddec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember_019edbd5-e61b-7933-8d19-83f946971ded" xlink:href="ck1437958-20260630.xsd#ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7ce1-9d68-7dd84b46ddec" xlink:to="loc_ck1437958_ConstructionLandAndLandDevelopmentPortfolioSegmentMember_019edbd5-e61b-7933-8d19-83f946971ded" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-74a1-8bb2-fa0c81eaf2ba" xlink:href="ck1437958-20260630.xsd#ck1437958_ResidentialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7ce1-9d68-7dd84b46ddec" xlink:to="loc_ck1437958_ResidentialRealEstateLoansMember_019edbd5-e61b-74a1-8bb2-fa0c81eaf2ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialRealEstateLoansMember_019edbd5-e61b-75f0-87a8-22dc186ba84f" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialRealEstateLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019edbd5-e61b-7ce1-9d68-7dd84b46ddec" xlink:to="loc_ck1437958_CommercialRealEstateLoansMember_019edbd5-e61b-75f0-87a8-22dc186ba84f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7da1-8e98-2760608a83d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-73bd-958b-36a6a8f1a771" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7da1-8e98-2760608a83d4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e61b-7da1-8e98-2760608a83d4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7da1-8e98-2760608a83d4" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e61b-7da1-8e98-2760608a83d4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e61b-7a01-a70d-a1ba0a99b17e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e61b-7da1-8e98-2760608a83d4" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e61b-7a01-a70d-a1ba0a99b17e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_RealEstateCollateralDependentLoansMember_019edbd5-e61b-7ebf-84cf-0205dd1a27e5" xlink:href="ck1437958-20260630.xsd#ck1437958_RealEstateCollateralDependentLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e61b-7a01-a70d-a1ba0a99b17e" xlink:to="loc_ck1437958_RealEstateCollateralDependentLoansMember_019edbd5-e61b-7ebf-84cf-0205dd1a27e5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_BusinessAssetsMember_019edbd5-e61b-754e-808c-f879f8f53f30" xlink:href="ck1437958-20260630.xsd#ck1437958_BusinessAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e61b-7a01-a70d-a1ba0a99b17e" xlink:to="loc_ck1437958_BusinessAssetsMember_019edbd5-e61b-754e-808c-f879f8f53f30" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralAxis_019edbd5-e61b-7ce8-91d0-03fc226fc381" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFinancingReceivableAllowanceForCreditLossesTable_019edbd5-e61b-73bd-958b-36a6a8f1a771" xlink:to="loc_us-gaap_CollateralAxis_019edbd5-e61b-7ce8-91d0-03fc226fc381" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralDomain_019edbd5-e61b-7ce8-91d0-03fc226fc381_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CollateralAxis_019edbd5-e61b-7ce8-91d0-03fc226fc381" xlink:to="loc_us-gaap_CollateralDomain_019edbd5-e61b-7ce8-91d0-03fc226fc381_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralDomain_019edbd5-e61b-76e3-b900-739eca4a568d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CollateralAxis_019edbd5-e61b-7ce8-91d0-03fc226fc381" xlink:to="loc_us-gaap_CollateralDomain_019edbd5-e61b-76e3-b900-739eca4a568d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CollateralDependentLoansMember_019edbd5-e61b-7ec0-a32a-02dcbdc25a5a" xlink:href="ck1437958-20260630.xsd#ck1437958_CollateralDependentLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019edbd5-e61b-76e3-b900-739eca4a568d" xlink:to="loc_ck1437958_CollateralDependentLoansMember_019edbd5-e61b-7ec0-a32a-02dcbdc25a5a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment_019edbd5-e61b-786a-bd83-7d79d4f56c06" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-7744-9143-f7523be8889a" xlink:to="loc_us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment_019edbd5-e61b-786a-bd83-7d79d4f56c06" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest_019edbd5-e61b-7b3c-bbb0-f7d78f4a30f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems_019edbd5-e61b-7744-9143-f7523be8889a" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest_019edbd5-e61b-7b3c-bbb0-f7d78f4a30f2" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/DepositsAdditionalInformationDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#DepositsAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/DepositsAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepositLiabilityLineItems_019edbd5-e61b-7188-bc97-ad6031c5d766" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepositLiabilityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimeDepositsTable_019edbd5-e61b-770c-aedb-a1833f09336b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TimeDepositsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DepositLiabilityLineItems_019edbd5-e61b-7188-bc97-ad6031c5d766" xlink:to="loc_us-gaap_TimeDepositsTable_019edbd5-e61b-770c-aedb-a1833f09336b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019edbd5-e61b-71ce-9887-693e2b725d63" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_TimeDepositsTable_019edbd5-e61b-770c-aedb-a1833f09336b" xlink:to="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019edbd5-e61b-71ce-9887-693e2b725d63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-71ce-9887-693e2b725d63_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019edbd5-e61b-71ce-9887-693e2b725d63" xlink:to="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-71ce-9887-693e2b725d63_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-7699-bcde-494d929da803" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019edbd5-e61b-71ce-9887-693e2b725d63" xlink:to="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-7699-bcde-494d929da803" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ReciprocalNOWAndMoneyMarketAccountsMember_019edbd5-e61b-7526-a6c7-e1e04dda167e" xlink:href="ck1437958-20260630.xsd#ck1437958_ReciprocalNOWAndMoneyMarketAccountsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019edbd5-e61b-7699-bcde-494d929da803" xlink:to="loc_ck1437958_ReciprocalNOWAndMoneyMarketAccountsMember_019edbd5-e61b-7526-a6c7-e1e04dda167e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019fcdb3-ae90-7f94-93e1-93ea23bf5423" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_TimeDepositsTable_019edbd5-e61b-770c-aedb-a1833f09336b" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019fcdb3-ae90-7f94-93e1-93ea23bf5423" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019fcdb3-ae90-7f94-93e1-93ea23bf5423_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019fcdb3-ae90-7f94-93e1-93ea23bf5423" xlink:to="loc_us-gaap_SegmentDomain_019fcdb3-ae90-7f94-93e1-93ea23bf5423_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019fcdb3-ae90-7851-8b7a-80960934f021" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019fcdb3-ae90-7f94-93e1-93ea23bf5423" xlink:to="loc_us-gaap_SegmentDomain_019fcdb3-ae90-7851-8b7a-80960934f021" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXMember_019fcdb3-d29d-7e08-95f0-f657e1a63333" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019fcdb3-ae90-7851-8b7a-80960934f021" xlink:to="loc_ck1437958_CCBXMember_019fcdb3-d29d-7e08-95f0-f657e1a63333" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Deposits_019edbd5-e61b-7be0-8aec-8fd3cc22a2f7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Deposits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DepositLiabilityLineItems_019edbd5-e61b-7188-bc97-ad6031c5d766" xlink:to="loc_us-gaap_Deposits_019edbd5-e61b-7be0-8aec-8fd3cc22a2f7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestAndFeeIncomeLoansAndLeases_019fcde6-ba13-79b6-b427-ea2c9dfb490d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestAndFeeIncomeLoansAndLeases"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DepositLiabilityLineItems_019edbd5-e61b-7188-bc97-ad6031c5d766" xlink:to="loc_us-gaap_InterestAndFeeIncomeLoansAndLeases_019fcde6-ba13-79b6-b427-ea2c9dfb490d" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/LeasesAdditionalInformationDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#LeasesAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/LeasesAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_019edbd5-e758-7902-be4a-e2783787a98f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_019edbd5-e61b-7eb1-bf84-8b773cd2f191" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019edbd5-e758-7902-be4a-e2783787a98f" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_019edbd5-e61b-7eb1-bf84-8b773cd2f191" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019edbd5-e61b-78f1-bc64-741ea8421468" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_019edbd5-e61b-7eb1-bf84-8b773cd2f191" xlink:to="loc_srt_RangeAxis_019edbd5-e61b-78f1-bc64-741ea8421468" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019edbd5-e61b-78f1-bc64-741ea8421468_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019edbd5-e61b-78f1-bc64-741ea8421468" xlink:to="loc_srt_RangeMember_019edbd5-e61b-78f1-bc64-741ea8421468_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019edbd5-e61b-758f-af1b-2467abdb2c3e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019edbd5-e61b-78f1-bc64-741ea8421468" xlink:to="loc_srt_RangeMember_019edbd5-e61b-758f-af1b-2467abdb2c3e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019edbd5-e61b-706c-99f8-f2a74ecd10cf" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019edbd5-e61b-758f-af1b-2467abdb2c3e" xlink:to="loc_srt_MinimumMember_019edbd5-e61b-706c-99f8-f2a74ecd10cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019edbd5-e758-7493-b44d-89ae09717a7f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019edbd5-e61b-758f-af1b-2467abdb2c3e" xlink:to="loc_srt_MaximumMember_019edbd5-e758-7493-b44d-89ae09717a7f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_019edbd5-e758-7ae1-ad5c-74c151adb8ca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019edbd5-e758-7902-be4a-e2783787a98f" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_019edbd5-e758-7ae1-ad5c-74c151adb8ca" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent_019edbd5-e758-75cd-923a-799b13f98a69" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019edbd5-e758-7902-be4a-e2783787a98f" xlink:to="loc_us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent_019edbd5-e758-75cd-923a-799b13f98a69" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_019edbd5-e758-7cc8-9156-c2283b69486d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeOperatingLeaseTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019edbd5-e758-7902-be4a-e2783787a98f" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_019edbd5-e758-7cc8-9156-c2283b69486d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_019edbd5-e758-7265-9c49-2031f0486199" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeOperatingLeaseRenewalTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019edbd5-e758-7902-be4a-e2783787a98f" xlink:to="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_019edbd5-e758-7265-9c49-2031f0486199" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_019edbd5-e758-7ee0-afa3-cd2bfdeb63e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019edbd5-e758-7902-be4a-e2783787a98f" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_019edbd5-e758-7ee0-afa3-cd2bfdeb63e7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseWeightedAverageRemainingLeaseTerm1_019edbd5-e758-724f-88c5-374579cbd15a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseWeightedAverageRemainingLeaseTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019edbd5-e758-7902-be4a-e2783787a98f" xlink:to="loc_us-gaap_FinanceLeaseWeightedAverageRemainingLeaseTerm1_019edbd5-e758-724f-88c5-374579cbd15a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseExpense_019edbd5-e758-79dc-a280-47b3b16c8050" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019edbd5-e758-7902-be4a-e2783787a98f" xlink:to="loc_us-gaap_OperatingLeaseExpense_019edbd5-e758-79dc-a280-47b3b16c8050" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseRightOfUseAssetAmortization_019edbd5-e758-7a47-af0a-113609ec60d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseRightOfUseAssetAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019edbd5-e758-7902-be4a-e2783787a98f" xlink:to="loc_us-gaap_FinanceLeaseRightOfUseAssetAmortization_019edbd5-e758-7a47-af0a-113609ec60d9" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseInterestExpense_019edbd5-e758-79ed-90ee-17df806a9aa3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseInterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019edbd5-e758-7902-be4a-e2783787a98f" xlink:to="loc_us-gaap_FinanceLeaseInterestExpense_019edbd5-e758-79ed-90ee-17df806a9aa3" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/StockBasedCompensationAdditionalInformationDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#StockBasedCompensationAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/StockBasedCompensationAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-773c-9bd8-0fafdf5c1bcd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019edbd5-e758-7102-aa88-4a9c2114e512" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-773c-9bd8-0fafdf5c1bcd" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019edbd5-e758-7102-aa88-4a9c2114e512" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019edbd5-e758-7091-97da-c886cfae9dab" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019edbd5-e758-7102-aa88-4a9c2114e512" xlink:to="loc_srt_RangeAxis_019edbd5-e758-7091-97da-c886cfae9dab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019edbd5-e758-7091-97da-c886cfae9dab_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019edbd5-e758-7091-97da-c886cfae9dab" xlink:to="loc_srt_RangeMember_019edbd5-e758-7091-97da-c886cfae9dab_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019edbd5-e758-7364-8933-bc9128dd353c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019edbd5-e758-7091-97da-c886cfae9dab" xlink:to="loc_srt_RangeMember_019edbd5-e758-7364-8933-bc9128dd353c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019edbd5-e758-7a53-859b-bcd7f743ff2d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019edbd5-e758-7364-8933-bc9128dd353c" xlink:to="loc_srt_MinimumMember_019edbd5-e758-7a53-859b-bcd7f743ff2d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019edbd5-e758-71ad-bb2a-3dd566b2f5e2" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019edbd5-e758-7364-8933-bc9128dd353c" xlink:to="loc_srt_MaximumMember_019edbd5-e758-71ad-bb2a-3dd566b2f5e2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019edbd5-e758-7fb8-8eb5-cb652c80e03a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019edbd5-e758-7102-aa88-4a9c2114e512" xlink:to="loc_us-gaap_PlanNameAxis_019edbd5-e758-7fb8-8eb5-cb652c80e03a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019edbd5-e758-7fb8-8eb5-cb652c80e03a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019edbd5-e758-7fb8-8eb5-cb652c80e03a" xlink:to="loc_us-gaap_PlanNameDomain_019edbd5-e758-7fb8-8eb5-cb652c80e03a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019edbd5-e758-7127-913b-f58e03278962" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019edbd5-e758-7fb8-8eb5-cb652c80e03a" xlink:to="loc_us-gaap_PlanNameDomain_019edbd5-e758-7127-913b-f58e03278962" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_TwoThousandAndEighteenOmnibusIncentivePlanMember_019edbd5-e758-738c-9486-eee87c215e95" xlink:href="ck1437958-20260630.xsd#ck1437958_TwoThousandAndEighteenOmnibusIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019edbd5-e758-7127-913b-f58e03278962" xlink:to="loc_ck1437958_TwoThousandAndEighteenOmnibusIncentivePlanMember_019edbd5-e758-738c-9486-eee87c215e95" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_DirectorsStockCompensationPlanMember_019edbd5-e758-7081-80f8-d38d7c5c7d60" xlink:href="ck1437958-20260630.xsd#ck1437958_DirectorsStockCompensationPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019edbd5-e758-7127-913b-f58e03278962" xlink:to="loc_ck1437958_DirectorsStockCompensationPlanMember_019edbd5-e758-7081-80f8-d38d7c5c7d60" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019edbd5-e758-70e2-b936-475286647905" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019edbd5-e758-7102-aa88-4a9c2114e512" xlink:to="loc_us-gaap_AwardTypeAxis_019edbd5-e758-70e2-b936-475286647905" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019edbd5-e758-70e2-b936-475286647905_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019edbd5-e758-70e2-b936-475286647905" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019edbd5-e758-70e2-b936-475286647905_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019edbd5-e758-7895-9c1c-7a5fd90dfb66" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019edbd5-e758-70e2-b936-475286647905" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019edbd5-e758-7895-9c1c-7a5fd90dfb66" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019edbd5-e758-74eb-b7b6-d27b5e54b939" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019edbd5-e758-7895-9c1c-7a5fd90dfb66" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019edbd5-e758-74eb-b7b6-d27b5e54b939" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019edbd5-e758-7310-a7b9-99392ae4d64d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019edbd5-e758-7895-9c1c-7a5fd90dfb66" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019edbd5-e758-7310-a7b9-99392ae4d64d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_019edbd5-e758-79dc-9013-6fe4ea4c317c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019edbd5-e758-7895-9c1c-7a5fd90dfb66" xlink:to="loc_us-gaap_RestrictedStockMember_019edbd5-e758-79dc-9013-6fe4ea4c317c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_019edbd5-e758-7f65-9bfc-f877a5712c40" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019edbd5-e758-7102-aa88-4a9c2114e512" xlink:to="loc_srt_TitleOfIndividualAxis_019edbd5-e758-7f65-9bfc-f877a5712c40" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019edbd5-e758-7f65-9bfc-f877a5712c40_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_019edbd5-e758-7f65-9bfc-f877a5712c40" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019edbd5-e758-7f65-9bfc-f877a5712c40_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019edbd5-e758-7f98-a01f-22109dd6726d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_019edbd5-e758-7f65-9bfc-f877a5712c40" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019edbd5-e758-7f98-a01f-22109dd6726d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_DirectorMember_019edbd5-e758-7ee0-9f19-52dd5553182d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_DirectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019edbd5-e758-7f98-a01f-22109dd6726d" xlink:to="loc_srt_DirectorMember_019edbd5-e758-7ee0-9f19-52dd5553182d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_BoardChairMember_019edbd5-e758-71ff-945a-e259673c2184" xlink:href="ck1437958-20260630.xsd#ck1437958_BoardChairMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019edbd5-e758-7f98-a01f-22109dd6726d" xlink:to="loc_ck1437958_BoardChairMember_019edbd5-e758-71ff-945a-e259673c2184" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommitteeChairsMember_019edbd5-e758-7a7a-96da-ef482a887b57" xlink:href="ck1437958-20260630.xsd#ck1437958_CommitteeChairsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019edbd5-e758-7f98-a01f-22109dd6726d" xlink:to="loc_ck1437958_CommitteeChairsMember_019edbd5-e758-7a7a-96da-ef482a887b57" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_NonFinancialRiskAndCompensationCommitteeChairMember_019edbd5-e758-760e-ad14-a4933dc83d3c" xlink:href="ck1437958-20260630.xsd#ck1437958_NonFinancialRiskAndCompensationCommitteeChairMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019edbd5-e758-7f98-a01f-22109dd6726d" xlink:to="loc_ck1437958_NonFinancialRiskAndCompensationCommitteeChairMember_019edbd5-e758-760e-ad14-a4933dc83d3c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_AssetLiabilityInvestmentCreditAndNominatingGovernanceChairMember_019edbd5-e758-770f-b2e3-dfef9d006277" xlink:href="ck1437958-20260630.xsd#ck1437958_AssetLiabilityInvestmentCreditAndNominatingGovernanceChairMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019edbd5-e758-7f98-a01f-22109dd6726d" xlink:to="loc_ck1437958_AssetLiabilityInvestmentCreditAndNominatingGovernanceChairMember_019edbd5-e758-770f-b2e3-dfef9d006277" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019edbd5-e758-705b-831d-f9cd62bfc96c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-773c-9bd8-0fafdf5c1bcd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019edbd5-e758-705b-831d-f9cd62bfc96c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_019edbd5-e758-7153-b497-d3576c1f8b01" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-773c-9bd8-0fafdf5c1bcd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_019edbd5-e758-7153-b497-d3576c1f8b01" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_019edbd5-e758-79f0-aee6-4c52d337ad94" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-773c-9bd8-0fafdf5c1bcd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_019edbd5-e758-79f0-aee6-4c52d337ad94" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_019edbd5-e758-7c0b-ab66-4597a16a24f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-773c-9bd8-0fafdf5c1bcd" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_019edbd5-e758-7c0b-ab66-4597a16a24f0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019edbd5-e758-7ada-8ac1-ed4ab5b721be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-773c-9bd8-0fafdf5c1bcd" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019edbd5-e758-7ada-8ac1-ed4ab5b721be" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_019edbd5-e758-79f4-b414-62e8e013ac0c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-773c-9bd8-0fafdf5c1bcd" xlink:to="loc_us-gaap_ShareBasedCompensation_019edbd5-e758-79f4-b414-62e8e013ac0c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019edbd5-e758-70f7-9742-551e9d3fb265" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-773c-9bd8-0fafdf5c1bcd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019edbd5-e758-70f7-9742-551e9d3fb265" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019edbd5-e758-7a95-8175-81d93459c4f3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-773c-9bd8-0fafdf5c1bcd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019edbd5-e758-7a95-8175-81d93459c4f3" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_019edbd5-e758-77bc-afaf-17b9678dd861" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-773c-9bd8-0fafdf5c1bcd" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_019edbd5-e758-77bc-afaf-17b9678dd861" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_MarketValueOfSharesGrantsPerYear_019edbd5-e758-71f0-bef7-07ebd4d3c478" xlink:href="ck1437958-20260630.xsd#ck1437958_MarketValueOfSharesGrantsPerYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-773c-9bd8-0fafdf5c1bcd" xlink:to="loc_ck1437958_MarketValueOfSharesGrantsPerYear_019edbd5-e758-71f0-bef7-07ebd4d3c478" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/StockBasedCompensationSummaryofStockOptionActivityDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#StockBasedCompensationSummaryofStockOptionActivityDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/StockBasedCompensationSummaryofStockOptionActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-7513-80f8-a028b130f817" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019edbd5-e758-7711-a955-53a814ba031b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-7513-80f8-a028b130f817" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019edbd5-e758-7711-a955-53a814ba031b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019edbd5-e758-768d-8de9-401f586e2321" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019edbd5-e758-7711-a955-53a814ba031b" xlink:to="loc_us-gaap_PlanNameAxis_019edbd5-e758-768d-8de9-401f586e2321" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019edbd5-e758-768d-8de9-401f586e2321_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019edbd5-e758-768d-8de9-401f586e2321" xlink:to="loc_us-gaap_PlanNameDomain_019edbd5-e758-768d-8de9-401f586e2321_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019edbd5-e758-711c-97aa-919b9dd71e2e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019edbd5-e758-768d-8de9-401f586e2321" xlink:to="loc_us-gaap_PlanNameDomain_019edbd5-e758-711c-97aa-919b9dd71e2e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_TwoThousandAndEighteenOmnibusIncentivePlanMember_019edbd5-e758-7651-8ed4-3766d3b597d5" xlink:href="ck1437958-20260630.xsd#ck1437958_TwoThousandAndEighteenOmnibusIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019edbd5-e758-711c-97aa-919b9dd71e2e" xlink:to="loc_ck1437958_TwoThousandAndEighteenOmnibusIncentivePlanMember_019edbd5-e758-7651-8ed4-3766d3b597d5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019edbd5-e758-7a6c-9240-4ff4ad0faed7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-7513-80f8-a028b130f817" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019edbd5-e758-7a6c-9240-4ff4ad0faed7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019edbd5-e758-7301-93ae-c175ff6193d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019edbd5-e758-7a6c-9240-4ff4ad0faed7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019edbd5-e758-7301-93ae-c175ff6193d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_019edbd5-e758-78a5-af93-682e5f7574c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019edbd5-e758-7a6c-9240-4ff4ad0faed7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_019edbd5-e758-78a5-af93-682e5f7574c4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019edbd5-e758-7a9a-a43a-09d89899b154" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019edbd5-e758-7a6c-9240-4ff4ad0faed7" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019edbd5-e758-7a9a-a43a-09d89899b154" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_019edbd5-e758-7de8-a325-2f9834c720f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019edbd5-e758-7a6c-9240-4ff4ad0faed7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_019edbd5-e758-7de8-a325-2f9834c720f6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_019edbd5-e758-7bb9-b402-6242b824075d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019edbd5-e758-7a6c-9240-4ff4ad0faed7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_019edbd5-e758-7bb9-b402-6242b824075d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019edbd5-e758-72b6-b930-48b4498167a8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_019edbd5-e758-7e72-9c0b-185692e75746" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-7513-80f8-a028b130f817" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_019edbd5-e758-7e72-9c0b-185692e75746" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_019edbd5-e758-7bd2-8d2c-14583b7e51ca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-7513-80f8-a028b130f817" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_019edbd5-e758-7bd2-8d2c-14583b7e51ca" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019edbd5-e758-7c93-89a4-e61fea4f5999" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-7513-80f8-a028b130f817" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019edbd5-e758-7c93-89a4-e61fea4f5999" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_019edbd5-e758-7006-9504-38aca824cffa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019edbd5-e758-7c93-89a4-e61fea4f5999" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_019edbd5-e758-7006-9504-38aca824cffa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_019edbd5-e758-76e0-bf74-56ace06eba74" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019edbd5-e758-7c93-89a4-e61fea4f5999" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_019edbd5-e758-76e0-bf74-56ace06eba74" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_019edbd5-e758-7a94-9067-fb3830137975" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019edbd5-e758-7c93-89a4-e61fea4f5999" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_019edbd5-e758-7a94-9067-fb3830137975" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice_019edbd5-e758-74af-8ba9-bba4dc1b252b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019edbd5-e758-7c93-89a4-e61fea4f5999" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice_019edbd5-e758-74af-8ba9-bba4dc1b252b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_019edbd5-e758-753b-981d-8c8ebbe99ac9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019edbd5-e758-7c93-89a4-e61fea4f5999" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_019edbd5-e758-753b-981d-8c8ebbe99ac9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_019edbd5-e758-718d-9182-cb7e9c5373ea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_019edbd5-e758-7b97-9ccd-ee697ccff93b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-7513-80f8-a028b130f817" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_019edbd5-e758-7b97-9ccd-ee697ccff93b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_019edbd5-e758-76b8-a481-5124b870e64f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-7513-80f8-a028b130f817" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_019edbd5-e758-76b8-a481-5124b870e64f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019edbd5-e758-7e01-ad5d-c4b915834337" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-7513-80f8-a028b130f817" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019edbd5-e758-7e01-ad5d-c4b915834337" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_019edbd5-e758-7bac-8dbc-ce76305de57f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019edbd5-e758-7e01-ad5d-c4b915834337" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_019edbd5-e758-7bac-8dbc-ce76305de57f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_019edbd5-e758-782a-a127-ec801b48eaa7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_019edbd5-e758-7644-a32d-86958319523a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019edbd5-e758-7e01-ad5d-c4b915834337" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_019edbd5-e758-7644-a32d-86958319523a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_019edbd5-e758-792f-99b0-c7f3f2703693" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019edbd5-e758-7e01-ad5d-c4b915834337" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_019edbd5-e758-792f-99b0-c7f3f2703693" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_019edbd5-e758-7e74-a016-b0e982351259" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019edbd5-e758-7e01-ad5d-c4b915834337" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_019edbd5-e758-7e74-a016-b0e982351259" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_019edbd5-e758-7599-8bfe-12a29f6546dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019edbd5-e758-7e01-ad5d-c4b915834337" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_019edbd5-e758-7599-8bfe-12a29f6546dd" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_019edbd5-e758-75af-be1c-37d934e62e82" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_019edbd5-e758-7d2f-8873-d2d2400e0e74" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019edbd5-e758-7e01-ad5d-c4b915834337" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_019edbd5-e758-7d2f-8873-d2d2400e0e74" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_019edbd5-e758-7bb1-95d3-445b0c56d671" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019edbd5-e758-7e01-ad5d-c4b915834337" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_019edbd5-e758-7bb1-95d3-445b0c56d671" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/StockBasedCompensationSummaryofNonvestedSharesDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#StockBasedCompensationSummaryofNonvestedSharesDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/StockBasedCompensationSummaryofNonvestedSharesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-728e-9445-82e108b9f041" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019edbd5-e758-7149-8740-79f88ee9902e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-728e-9445-82e108b9f041" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019edbd5-e758-7149-8740-79f88ee9902e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019edbd5-e758-7f5d-b109-5d72541bbc3a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019edbd5-e758-7149-8740-79f88ee9902e" xlink:to="loc_us-gaap_AwardTypeAxis_019edbd5-e758-7f5d-b109-5d72541bbc3a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019edbd5-e758-7f5d-b109-5d72541bbc3a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019edbd5-e758-7f5d-b109-5d72541bbc3a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019edbd5-e758-7f5d-b109-5d72541bbc3a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019edbd5-e758-72cd-b719-7a8874f8a534" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019edbd5-e758-7f5d-b109-5d72541bbc3a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019edbd5-e758-72cd-b719-7a8874f8a534" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019edbd5-e758-7ac0-aaec-ee378e1a0f92" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019edbd5-e758-72cd-b719-7a8874f8a534" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019edbd5-e758-7ac0-aaec-ee378e1a0f92" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_019edbd5-e758-7977-afa4-400e582b1e98" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019edbd5-e758-72cd-b719-7a8874f8a534" xlink:to="loc_us-gaap_RestrictedStockMember_019edbd5-e758-7977-afa4-400e582b1e98" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019edbd5-e758-7cce-87ee-dee92818b843" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019edbd5-e758-7149-8740-79f88ee9902e" xlink:to="loc_us-gaap_PlanNameAxis_019edbd5-e758-7cce-87ee-dee92818b843" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019edbd5-e758-7cce-87ee-dee92818b843_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019edbd5-e758-7cce-87ee-dee92818b843" xlink:to="loc_us-gaap_PlanNameDomain_019edbd5-e758-7cce-87ee-dee92818b843_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019edbd5-e758-7102-b4e6-f47896f09045" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019edbd5-e758-7cce-87ee-dee92818b843" xlink:to="loc_us-gaap_PlanNameDomain_019edbd5-e758-7102-b4e6-f47896f09045" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_DirectorsStockCompensationPlanMember_019edbd5-e758-7990-b0fc-b71419af08f8" xlink:href="ck1437958-20260630.xsd#ck1437958_DirectorsStockCompensationPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019edbd5-e758-7102-b4e6-f47896f09045" xlink:to="loc_ck1437958_DirectorsStockCompensationPlanMember_019edbd5-e758-7990-b0fc-b71419af08f8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019edbd5-e758-7076-81c5-74eb5c15f031" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-728e-9445-82e108b9f041" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019edbd5-e758-7076-81c5-74eb5c15f031" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019edbd5-e758-78bd-b0d7-4245be83b1dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019edbd5-e758-7076-81c5-74eb5c15f031" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019edbd5-e758-78bd-b0d7-4245be83b1dc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019edbd5-e758-772c-b400-5950bf134acf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019edbd5-e758-7076-81c5-74eb5c15f031" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019edbd5-e758-772c-b400-5950bf134acf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019edbd5-e758-71b7-81fb-43863143571f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019edbd5-e758-7076-81c5-74eb5c15f031" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019edbd5-e758-71b7-81fb-43863143571f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019edbd5-e758-7826-816f-8bbb9071a8a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019edbd5-e758-7076-81c5-74eb5c15f031" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019edbd5-e758-7826-816f-8bbb9071a8a4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019edbd5-e758-7d44-9cc0-b1337b09f687" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019edbd5-e758-78c8-80dc-43c88f98219e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019edbd5-e758-728e-9445-82e108b9f041" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019edbd5-e758-78c8-80dc-43c88f98219e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019edbd5-e758-7f66-a069-9c276a345873" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019edbd5-e758-78c8-80dc-43c88f98219e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019edbd5-e758-7f66-a069-9c276a345873" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019edbd5-e758-7b8d-b4b0-a175891a3f17" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019edbd5-e758-78c8-80dc-43c88f98219e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019edbd5-e758-7b8d-b4b0-a175891a3f17" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019edbd5-e758-73a9-8c1c-1eed5c265390" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019edbd5-e758-78c8-80dc-43c88f98219e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019edbd5-e758-73a9-8c1c-1eed5c265390" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019edbd5-e758-7092-a1bb-b1609e2b48f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019edbd5-e758-78c8-80dc-43c88f98219e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019edbd5-e758-7092-a1bb-b1609e2b48f4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019edbd5-e758-79ee-9819-8c7e2bfae710" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/FairValueMeasurementsSummaryofEstimatedFairValuesofFinancialInstrumentsDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#FairValueMeasurementsSummaryofEstimatedFairValuesofFinancialInstrumentsDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/FairValueMeasurementsSummaryofEstimatedFairValuesofFinancialInstrumentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019edbd5-e758-7c1d-8b5e-ea0bbc9032d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019edbd5-e758-7d4d-a706-a401b7bd9403" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByBalanceSheetGroupingTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019edbd5-e758-7c1d-8b5e-ea0bbc9032d7" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019edbd5-e758-7d4d-a706-a401b7bd9403" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_019edbd5-e758-7254-9ca0-faf8c5b6e52f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByMeasurementBasisAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019edbd5-e758-7d4d-a706-a401b7bd9403" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_019edbd5-e758-7254-9ca0-faf8c5b6e52f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019edbd5-e758-7254-9ca0-faf8c5b6e52f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019edbd5-e758-7254-9ca0-faf8c5b6e52f" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019edbd5-e758-7254-9ca0-faf8c5b6e52f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019edbd5-e758-7b9f-8e5d-47cc26953f4e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019edbd5-e758-7254-9ca0-faf8c5b6e52f" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019edbd5-e758-7b9f-8e5d-47cc26953f4e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019edbd5-e758-747d-a163-714c10d667d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019edbd5-e758-7b9f-8e5d-47cc26953f4e" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019edbd5-e758-747d-a163-714c10d667d1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_019edbd5-e758-7ef2-a145-ad9c89fbdfa6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019edbd5-e758-747d-a163-714c10d667d1" xlink:to="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_019edbd5-e758-7ef2-a145-ad9c89fbdfa6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019edbd5-e758-792a-a3fb-ace0be911f35" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019edbd5-e758-747d-a163-714c10d667d1" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019edbd5-e758-792a-a3fb-ace0be911f35" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-7281-89b0-8e22595b0bec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019edbd5-e758-7d4d-a706-a401b7bd9403" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-7281-89b0-8e22595b0bec" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7281-89b0-8e22595b0bec_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-7281-89b0-8e22595b0bec" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7281-89b0-8e22595b0bec_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7133-9612-19fdf86f48de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-7281-89b0-8e22595b0bec" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7133-9612-19fdf86f48de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019edbd5-e758-7ad8-b06c-9360461c1b82" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7133-9612-19fdf86f48de" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019edbd5-e758-7ad8-b06c-9360461c1b82" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019edbd5-e758-74b7-b023-e66d16c2a2d6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7133-9612-19fdf86f48de" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019edbd5-e758-74b7-b023-e66d16c2a2d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019edbd5-e758-7196-81da-40d8d9d7cd13" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7133-9612-19fdf86f48de" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019edbd5-e758-7196-81da-40d8d9d7cd13" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_019edbd5-e758-7b7f-ad7e-93ecfa984a00" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019edbd5-e758-7c1d-8b5e-ea0bbc9032d7" xlink:to="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_019edbd5-e758-7b7f-ad7e-93ecfa984a00" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019edbd5-e758-70d1-a4ed-079e5648bacc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_019edbd5-e758-7b7f-ad7e-93ecfa984a00" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019edbd5-e758-70d1-a4ed-079e5648bacc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_InterestBearingDepositInBanksFairValue_019edbd5-e758-790d-9f55-fc4dd91cf470" xlink:href="ck1437958-20260630.xsd#ck1437958_InterestBearingDepositInBanksFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_019edbd5-e758-7b7f-ad7e-93ecfa984a00" xlink:to="loc_ck1437958_InterestBearingDepositInBanksFairValue_019edbd5-e758-790d-9f55-fc4dd91cf470" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecurities_019edbd5-e758-7da3-ba16-9ae82f4d5b7c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_019edbd5-e758-7b7f-ad7e-93ecfa984a00" xlink:to="loc_us-gaap_DebtSecurities_019edbd5-e758-7da3-ba16-9ae82f4d5b7c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherInvestments_019edbd5-e758-7595-a2b4-c84615d2e223" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_019edbd5-e758-7b7f-ad7e-93ecfa984a00" xlink:to="loc_us-gaap_OtherInvestments_019edbd5-e758-7595-a2b4-c84615d2e223" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansHeldForSaleFairValueDisclosure_019edbd5-e758-7271-a829-405d53450ab3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansHeldForSaleFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_019edbd5-e758-7b7f-ad7e-93ecfa984a00" xlink:to="loc_us-gaap_LoansHeldForSaleFairValueDisclosure_019edbd5-e758-7271-a829-405d53450ab3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansReceivableFairValueDisclosure_019edbd5-e758-7167-8825-2a78ef825a25" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansReceivableFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_019edbd5-e758-7b7f-ad7e-93ecfa984a00" xlink:to="loc_us-gaap_LoansReceivableFairValueDisclosure_019edbd5-e758-7167-8825-2a78ef825a25" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestReceivable_019edbd5-e758-770c-8aed-8fdd845c5688" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_019edbd5-e758-7b7f-ad7e-93ecfa984a00" xlink:to="loc_us-gaap_InterestReceivable_019edbd5-e758-770c-8aed-8fdd845c5688" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract_019edbd5-e758-76d6-9cf0-c061b53743cd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019edbd5-e758-7c1d-8b5e-ea0bbc9032d7" xlink:to="loc_us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract_019edbd5-e758-76d6-9cf0-c061b53743cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepositsFairValueDisclosure_019edbd5-e758-7859-b8d2-f1449dc28e0e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepositsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract_019edbd5-e758-76d6-9cf0-c061b53743cd" xlink:to="loc_us-gaap_DepositsFairValueDisclosure_019edbd5-e758-7859-b8d2-f1449dc28e0e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations_019edbd5-e758-711e-9e08-a9225cd813de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract_019edbd5-e758-76d6-9cf0-c061b53743cd" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisSubordinatedDebtObligations_019edbd5-e758-711e-9e08-a9225cd813de" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust_019edbd5-e758-73aa-a55b-94230b2f2d80" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract_019edbd5-e758-76d6-9cf0-c061b53743cd" xlink:to="loc_us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust_019edbd5-e758-73aa-a55b-94230b2f2d80" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_AccruedInterestPayableFairValue_019edbd5-e758-77bf-aa4c-7c7e8f25b170" xlink:href="ck1437958-20260630.xsd#ck1437958_AccruedInterestPayableFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract_019edbd5-e758-76d6-9cf0-c061b53743cd" xlink:to="loc_ck1437958_AccruedInterestPayableFairValue_019edbd5-e758-77bf-aa4c-7c7e8f25b170" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/FairValueMeasurementsSummaryofAssetsMeasuredatFairValueonRecurringBasisDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#FairValueMeasurementsSummaryofAssetsMeasuredatFairValueonRecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/FairValueMeasurementsSummaryofAssetsMeasuredatFairValueonRecurringBasisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019edbd5-e758-7d26-8fb4-5595c82d22bf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019edbd5-e758-76d7-afd6-f1898314b49b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019edbd5-e758-7d26-8fb4-5595c82d22bf" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019edbd5-e758-76d7-afd6-f1898314b49b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_019edbd5-e758-7577-b0f2-5f6749af9e2b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByMeasurementBasisAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019edbd5-e758-76d7-afd6-f1898314b49b" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_019edbd5-e758-7577-b0f2-5f6749af9e2b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019edbd5-e758-7577-b0f2-5f6749af9e2b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019edbd5-e758-7577-b0f2-5f6749af9e2b" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019edbd5-e758-7577-b0f2-5f6749af9e2b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019edbd5-e758-7480-8ce7-0cc7d94d78ef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019edbd5-e758-7577-b0f2-5f6749af9e2b" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019edbd5-e758-7480-8ce7-0cc7d94d78ef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019edbd5-e758-7137-8d44-dc825c11e725" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019edbd5-e758-7480-8ce7-0cc7d94d78ef" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019edbd5-e758-7137-8d44-dc825c11e725" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019edbd5-e758-7bf6-8936-b0bf9237c612" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019edbd5-e758-7480-8ce7-0cc7d94d78ef" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019edbd5-e758-7bf6-8936-b0bf9237c612" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019edbd5-e758-7242-9968-b2089a0b22d0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019edbd5-e758-76d7-afd6-f1898314b49b" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019edbd5-e758-7242-9968-b2089a0b22d0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019edbd5-e758-7242-9968-b2089a0b22d0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019edbd5-e758-7242-9968-b2089a0b22d0" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019edbd5-e758-7242-9968-b2089a0b22d0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019edbd5-e758-7048-a589-673ad5bbaea3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019edbd5-e758-7242-9968-b2089a0b22d0" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019edbd5-e758-7048-a589-673ad5bbaea3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_019edbd5-e758-7fb5-8dcf-25d57b6390d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_019edbd5-e758-7048-a589-673ad5bbaea3" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_019edbd5-e758-7fb5-8dcf-25d57b6390d2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-76ca-8fc8-29599e9bc493" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019edbd5-e758-76d7-afd6-f1898314b49b" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-76ca-8fc8-29599e9bc493" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-76ca-8fc8-29599e9bc493_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-76ca-8fc8-29599e9bc493" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-76ca-8fc8-29599e9bc493_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7ef9-9f67-06602e03cde6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-76ca-8fc8-29599e9bc493" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7ef9-9f67-06602e03cde6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019edbd5-e758-78f6-88d9-672b709e838d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7ef9-9f67-06602e03cde6" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019edbd5-e758-78f6-88d9-672b709e838d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019edbd5-e758-7a89-9b81-25f68e26359a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7ef9-9f67-06602e03cde6" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019edbd5-e758-7a89-9b81-25f68e26359a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019edbd5-e758-7ae5-a4b0-2101c6b23de8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7ef9-9f67-06602e03cde6" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019edbd5-e758-7ae5-a4b0-2101c6b23de8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_019edbd5-e758-706a-b9b9-1b25abf885d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByAssetClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019edbd5-e758-76d7-afd6-f1898314b49b" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_019edbd5-e758-706a-b9b9-1b25abf885d7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019edbd5-e758-706a-b9b9-1b25abf885d7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019edbd5-e758-706a-b9b9-1b25abf885d7" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019edbd5-e758-706a-b9b9-1b25abf885d7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019edbd5-e758-7a3e-b74d-0d614b8a79c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019edbd5-e758-706a-b9b9-1b25abf885d7" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019edbd5-e758-7a3e-b74d-0d614b8a79c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019edbd5-e758-71ec-9045-04c26495766e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019edbd5-e758-7a3e-b74d-0d614b8a79c7" xlink:to="loc_us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember_019edbd5-e758-71ec-9045-04c26495766e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialMortgageBackedSecuritiesMember_019edbd5-e758-7e32-a98a-9c511e913d90" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResidentialMortgageBackedSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019edbd5-e758-7a3e-b74d-0d614b8a79c7" xlink:to="loc_us-gaap_ResidentialMortgageBackedSecuritiesMember_019edbd5-e758-7e32-a98a-9c511e913d90" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_019edbd5-e758-7dab-b044-514453791fe0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019edbd5-e758-7d26-8fb4-5595c82d22bf" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_019edbd5-e758-7dab-b044-514453791fe0" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/FairValueMeasurementsSummaryofFinancialAssetsandLiabilitiesMeasuredatFairValueonNonrecurringBasisDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#FairValueMeasurementsSummaryofFinancialAssetsandLiabilitiesMeasuredatFairValueonNonrecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/FairValueMeasurementsSummaryofFinancialAssetsandLiabilitiesMeasuredatFairValueonNonrecurringBasisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019edbd5-e758-7059-80d7-2628ddad989b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019edbd5-e758-72d2-9f13-41afaa506307" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019edbd5-e758-7059-80d7-2628ddad989b" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019edbd5-e758-72d2-9f13-41afaa506307" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019edbd5-e758-7a46-91d1-c8f5b6da96b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019edbd5-e758-72d2-9f13-41afaa506307" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019edbd5-e758-7a46-91d1-c8f5b6da96b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019edbd5-e758-7a46-91d1-c8f5b6da96b3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019edbd5-e758-7a46-91d1-c8f5b6da96b3" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019edbd5-e758-7a46-91d1-c8f5b6da96b3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019edbd5-e758-71fb-8375-6b0954897125" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019edbd5-e758-7a46-91d1-c8f5b6da96b3" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019edbd5-e758-71fb-8375-6b0954897125" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsNonrecurringMember_019edbd5-e758-7a8a-a908-1e12e81a1184" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsNonrecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_019edbd5-e758-71fb-8375-6b0954897125" xlink:to="loc_us-gaap_FairValueMeasurementsNonrecurringMember_019edbd5-e758-7a8a-a908-1e12e81a1184" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-789f-af96-f57715a2babb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019edbd5-e758-72d2-9f13-41afaa506307" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-789f-af96-f57715a2babb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-789f-af96-f57715a2babb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-789f-af96-f57715a2babb" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-789f-af96-f57715a2babb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7a23-9191-195baef51273" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-789f-af96-f57715a2babb" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7a23-9191-195baef51273" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019edbd5-e758-7393-81f6-6ef3d8ed61a9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7a23-9191-195baef51273" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019edbd5-e758-7393-81f6-6ef3d8ed61a9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019edbd5-e758-754f-b122-d49c9542245c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7a23-9191-195baef51273" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019edbd5-e758-754f-b122-d49c9542245c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019edbd5-e758-7ae5-9386-007aa388558c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7a23-9191-195baef51273" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019edbd5-e758-7ae5-9386-007aa388558c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_019edbd5-e758-7024-be5f-b2712f13588f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByAssetClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019edbd5-e758-72d2-9f13-41afaa506307" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_019edbd5-e758-7024-be5f-b2712f13588f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019edbd5-e758-7024-be5f-b2712f13588f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019edbd5-e758-7024-be5f-b2712f13588f" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019edbd5-e758-7024-be5f-b2712f13588f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019edbd5-e758-7f66-a5a1-7fe3b6c6f95c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019edbd5-e758-7024-be5f-b2712f13588f" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019edbd5-e758-7f66-a5a1-7fe3b6c6f95c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ImpairedLoansMember_019edbd5-e758-72e2-b76d-cb7fc6b1bc6e" xlink:href="ck1437958-20260630.xsd#ck1437958_ImpairedLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019edbd5-e758-7f66-a5a1-7fe3b6c6f95c" xlink:to="loc_ck1437958_ImpairedLoansMember_019edbd5-e758-72e2-b76d-cb7fc6b1bc6e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_019edbd5-e758-7483-9206-57df4c9f666b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019edbd5-e758-7f66-a5a1-7fe3b6c6f95c" xlink:to="loc_us-gaap_EquitySecuritiesMember_019edbd5-e758-7483-9206-57df4c9f666b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_019edbd5-e758-77aa-b399-572dcada143c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019edbd5-e758-7059-80d7-2628ddad989b" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_019edbd5-e758-77aa-b399-572dcada143c" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/FairValueMeasurementsSummaryofCarryingValueofEquitySecuritiesWithoutReadilyDeterminableFairValuesDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#FairValueMeasurementsSummaryofCarryingValueofEquitySecuritiesWithoutReadilyDeterminableFairValuesDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/FairValueMeasurementsSummaryofCarryingValueofEquitySecuritiesWithoutReadilyDeterminableFairValuesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueLineItems_019edbd5-e758-7089-a5ca-122cdde6f66e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueTable_019edbd5-e758-720e-8a97-02ab6bc95410" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueLineItems_019edbd5-e758-7089-a5ca-122cdde6f66e" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueTable_019edbd5-e758-720e-8a97-02ab6bc95410" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-7049-b940-63b675976f50" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueTable_019edbd5-e758-720e-8a97-02ab6bc95410" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-7049-b940-63b675976f50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7049-b940-63b675976f50_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-7049-b940-63b675976f50" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7049-b940-63b675976f50_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7963-b1fc-50ababf12507" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-7049-b940-63b675976f50" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7963-b1fc-50ababf12507" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019edbd5-e758-796c-85ea-551f2e0b111e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7963-b1fc-50ababf12507" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019edbd5-e758-796c-85ea-551f2e0b111e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeAxis_019edbd5-e758-7471-a0e6-b229bf6768af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueTable_019edbd5-e758-720e-8a97-02ab6bc95410" xlink:to="loc_us-gaap_InvestmentTypeAxis_019edbd5-e758-7471-a0e6-b229bf6768af" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_019edbd5-e758-7471-a0e6-b229bf6768af_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentTypeCategorizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_InvestmentTypeAxis_019edbd5-e758-7471-a0e6-b229bf6768af" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_019edbd5-e758-7471-a0e6-b229bf6768af_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_019edbd5-e758-7bde-96ef-20efb5bf7872" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentTypeCategorizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_InvestmentTypeAxis_019edbd5-e758-7471-a0e6-b229bf6768af" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_019edbd5-e758-7bde-96ef-20efb5bf7872" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_SpecializedBankTechnologyCompanyMember_019edbd5-e758-7b4c-9c24-98f129f8e88c" xlink:href="ck1437958-20260630.xsd#ck1437958_SpecializedBankTechnologyCompanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019edbd5-e758-7bde-96ef-20efb5bf7872" xlink:to="loc_ck1437958_SpecializedBankTechnologyCompanyMember_019edbd5-e758-7b4c-9c24-98f129f8e88c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_TechnologyCompany1Member_019edbd5-e758-7a4c-b25a-c9312dc89aa1" xlink:href="ck1437958-20260630.xsd#ck1437958_TechnologyCompany1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019edbd5-e758-7bde-96ef-20efb5bf7872" xlink:to="loc_ck1437958_TechnologyCompany1Member_019edbd5-e758-7a4c-b25a-c9312dc89aa1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_TechnologyCompany2Member_019edbd5-e758-74c6-8d01-f0f233ab69d9" xlink:href="ck1437958-20260630.xsd#ck1437958_TechnologyCompany2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019edbd5-e758-7bde-96ef-20efb5bf7872" xlink:to="loc_ck1437958_TechnologyCompany2Member_019edbd5-e758-74c6-8d01-f0f233ab69d9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_TechnologyCompany3Member_019edbd5-e758-7f29-9c38-eee854d22a61" xlink:href="ck1437958-20260630.xsd#ck1437958_TechnologyCompany3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019edbd5-e758-7bde-96ef-20efb5bf7872" xlink:to="loc_ck1437958_TechnologyCompany3Member_019edbd5-e758-7f29-9c38-eee854d22a61" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedAtCost_019edbd5-e758-7e20-86bb-77b4e896f6cf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentOwnedAtCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueLineItems_019edbd5-e758-7089-a5ca-122cdde6f66e" xlink:to="loc_us-gaap_InvestmentOwnedAtCost_019edbd5-e758-7e20-86bb-77b4e896f6cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019edbd5-e758-7ee5-bfdc-3b7c13023f78" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueLineItems_019edbd5-e758-7089-a5ca-122cdde6f66e" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019edbd5-e758-7ee5-bfdc-3b7c13023f78" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount_019edbd5-e758-79c5-8b46-e77f20a8a48d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019edbd5-e758-7ee5-bfdc-3b7c13023f78" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount_019edbd5-e758-79c5-8b46-e77f20a8a48d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_EquitySecuritiesWithoutReadilyDeterminableFairValuePurchases_019edbd5-e758-72c0-ba4b-db469f178bd3" xlink:href="ck1437958-20260630.xsd#ck1437958_EquitySecuritiesWithoutReadilyDeterminableFairValuePurchases"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019edbd5-e758-7ee5-bfdc-3b7c13023f78" xlink:to="loc_ck1437958_EquitySecuritiesWithoutReadilyDeterminableFairValuePurchases_019edbd5-e758-72c0-ba4b-db469f178bd3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_EquitySecuritiesWithoutReadilyDeterminableFairValueAdditionOfEquitySecurities_019edbd5-e758-7a20-ae8c-365b08b9047a" xlink:href="ck1437958-20260630.xsd#ck1437958_EquitySecuritiesWithoutReadilyDeterminableFairValueAdditionOfEquitySecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019edbd5-e758-7ee5-bfdc-3b7c13023f78" xlink:to="loc_ck1437958_EquitySecuritiesWithoutReadilyDeterminableFairValueAdditionOfEquitySecurities_019edbd5-e758-7a20-ae8c-365b08b9047a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount_019edbd5-e758-706c-91ad-10ee9db035f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/FairValueMeasurementsSummaryofAssetsandLiabilitiesClassifiedasLevel3andMeasuredatFairValueonNonrecurringBasisDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#FairValueMeasurementsSummaryofAssetsandLiabilitiesClassifiedasLevel3andMeasuredatFairValueonNonrecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/FairValueMeasurementsSummaryofAssetsandLiabilitiesClassifiedasLevel3andMeasuredatFairValueonNonrecurringBasisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019edbd5-e758-70e1-bbe3-e73bb8782794" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019edbd5-e758-7dc6-95fa-485fea7c0fdd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019edbd5-e758-70e1-bbe3-e73bb8782794" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019edbd5-e758-7dc6-95fa-485fea7c0fdd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-7448-9c37-025024ae578f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019edbd5-e758-7dc6-95fa-485fea7c0fdd" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-7448-9c37-025024ae578f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7448-9c37-025024ae578f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-7448-9c37-025024ae578f" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-7448-9c37-025024ae578f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-79e3-9e09-6f12755281b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019edbd5-e758-7448-9c37-025024ae578f" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-79e3-9e09-6f12755281b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019edbd5-e758-7be9-8f39-192a05c2bdf2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019edbd5-e758-79e3-9e09-6f12755281b1" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019edbd5-e758-7be9-8f39-192a05c2bdf2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_019edbd5-e758-760f-a29b-5027a42f8414" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019edbd5-e758-7dc6-95fa-485fea7c0fdd" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_019edbd5-e758-760f-a29b-5027a42f8414" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_019edbd5-e758-760f-a29b-5027a42f8414_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_019edbd5-e758-760f-a29b-5027a42f8414" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_019edbd5-e758-760f-a29b-5027a42f8414_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_019edbd5-e758-7530-bbdc-ba6061379b67" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_019edbd5-e758-760f-a29b-5027a42f8414" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_019edbd5-e758-7530-bbdc-ba6061379b67" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputDiscountRateMember_019edbd5-e758-746a-aa7c-9990e5b6d9dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputDiscountRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_019edbd5-e758-7530-bbdc-ba6061379b67" xlink:to="loc_us-gaap_MeasurementInputDiscountRateMember_019edbd5-e758-746a-aa7c-9990e5b6d9dd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueAxis_019edbd5-e758-7a59-9ff9-536b070bc4d5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ValuationTechniqueAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019edbd5-e758-7dc6-95fa-485fea7c0fdd" xlink:to="loc_us-gaap_ValuationTechniqueAxis_019edbd5-e758-7a59-9ff9-536b070bc4d5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueDomain_019edbd5-e758-7a59-9ff9-536b070bc4d5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ValuationTechniqueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ValuationTechniqueAxis_019edbd5-e758-7a59-9ff9-536b070bc4d5" xlink:to="loc_us-gaap_ValuationTechniqueDomain_019edbd5-e758-7a59-9ff9-536b070bc4d5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueDomain_019edbd5-e758-749d-ba2a-4d94397b801b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ValuationTechniqueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ValuationTechniqueAxis_019edbd5-e758-7a59-9ff9-536b070bc4d5" xlink:to="loc_us-gaap_ValuationTechniqueDomain_019edbd5-e758-749d-ba2a-4d94397b801b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketApproachValuationTechniqueMember_019edbd5-e758-7e7e-bed5-b04e977aa164" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MarketApproachValuationTechniqueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationTechniqueDomain_019edbd5-e758-749d-ba2a-4d94397b801b" xlink:to="loc_us-gaap_MarketApproachValuationTechniqueMember_019edbd5-e758-7e7e-bed5-b04e977aa164" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ImpairedLoansMeasurementInput_019edbd5-e758-74c6-bd13-19b4ba10ace3" xlink:href="ck1437958-20260630.xsd#ck1437958_ImpairedLoansMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019edbd5-e758-70e1-bbe3-e73bb8782794" xlink:to="loc_ck1437958_ImpairedLoansMeasurementInput_019edbd5-e758-74c6-bd13-19b4ba10ace3" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/LossEarningsPerCommonShareScheduleofComputationofBasicandDilutedEarningsPerCommonShareDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#LossEarningsPerCommonShareScheduleofComputationofBasicandDilutedEarningsPerCommonShareDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/LossEarningsPerCommonShareScheduleofComputationofBasicandDilutedEarningsPerCommonShareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019edbd5-e758-7063-8870-e56b0b9e32db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019edbd5-e758-765e-a6d6-2152b3834586" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019edbd5-e758-7063-8870-e56b0b9e32db" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019edbd5-e758-765e-a6d6-2152b3834586" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019edbd5-e758-7aa3-97a5-db8be0779c55" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019edbd5-e758-765e-a6d6-2152b3834586" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019edbd5-e758-7aa3-97a5-db8be0779c55" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019edbd5-e758-7aa3-97a5-db8be0779c55_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019edbd5-e758-7aa3-97a5-db8be0779c55" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019edbd5-e758-7aa3-97a5-db8be0779c55_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019edbd5-e758-75ae-8bd1-b9a03ef3d73f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019edbd5-e758-7aa3-97a5-db8be0779c55" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019edbd5-e758-75ae-8bd1-b9a03ef3d73f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_StockOptionsAndRestrictedStockMember_019edbd5-e758-73a2-84a2-2a772b2c5ddc" xlink:href="ck1437958-20260630.xsd#ck1437958_StockOptionsAndRestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019edbd5-e758-75ae-8bd1-b9a03ef3d73f" xlink:to="loc_ck1437958_StockOptionsAndRestrictedStockMember_019edbd5-e758-73a2-84a2-2a772b2c5ddc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_019edbd5-e758-7251-a925-8f970dad1f2f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019edbd5-e758-7063-8870-e56b0b9e32db" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_019edbd5-e758-7251-a925-8f970dad1f2f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019edbd5-e758-75b9-89e6-2b74dd22ff96" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_019edbd5-e758-7251-a925-8f970dad1f2f" xlink:to="loc_us-gaap_NetIncomeLoss_019edbd5-e758-75b9-89e6-2b74dd22ff96" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019edbd5-e758-7dc1-b040-9f358698041f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_019edbd5-e758-7251-a925-8f970dad1f2f" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019edbd5-e758-7dc1-b040-9f358698041f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_019edbd5-e758-789c-9ec4-3c9a5ef99e40" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_019edbd5-e758-7251-a925-8f970dad1f2f" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_019edbd5-e758-789c-9ec4-3c9a5ef99e40" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019edbd5-e758-7816-8340-4be6909eda1c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_019edbd5-e758-7251-a925-8f970dad1f2f" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019edbd5-e758-7816-8340-4be6909eda1c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_019edbd5-e758-7727-bf9a-93fc9f1dac43" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_019edbd5-e758-7251-a925-8f970dad1f2f" xlink:to="loc_us-gaap_EarningsPerShareBasic_019edbd5-e758-7727-bf9a-93fc9f1dac43" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_019edbd5-e758-7035-bbe4-1e7624445be9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_019edbd5-e758-7251-a925-8f970dad1f2f" xlink:to="loc_us-gaap_EarningsPerShareDiluted_019edbd5-e758-7035-bbe4-1e7624445be9" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019edbd5-e758-741f-ae40-24e35730a5df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_019edbd5-e758-7251-a925-8f970dad1f2f" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019edbd5-e758-741f-ae40-24e35730a5df" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/SegmentReportingAdditionalInformationDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#SegmentReportingAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/SegmentReportingAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7583-9501-b44312979315" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019edbd5-e758-7ff6-b388-c471fe7c39c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7583-9501-b44312979315" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019edbd5-e758-7ff6-b388-c471fe7c39c7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019edbd5-e758-723b-8a2e-2d7064fc45f0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019edbd5-e758-7ff6-b388-c471fe7c39c7" xlink:to="loc_srt_StatementGeographicalAxis_019edbd5-e758-723b-8a2e-2d7064fc45f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019edbd5-e758-723b-8a2e-2d7064fc45f0_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019edbd5-e758-723b-8a2e-2d7064fc45f0" xlink:to="loc_srt_SegmentGeographicalDomain_019edbd5-e758-723b-8a2e-2d7064fc45f0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019edbd5-e758-7a73-b788-81cfbe327965" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019edbd5-e758-723b-8a2e-2d7064fc45f0" xlink:to="loc_srt_SegmentGeographicalDomain_019edbd5-e758-7a73-b788-81cfbe327965" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_SnohomishCountyMember_019edbd5-e758-7daf-bd7c-da7f718bbc72" xlink:href="ck1437958-20260630.xsd#ck1437958_SnohomishCountyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019edbd5-e758-7a73-b788-81cfbe327965" xlink:to="loc_ck1437958_SnohomishCountyMember_019edbd5-e758-7daf-bd7c-da7f718bbc72" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_KingCountyMember_019edbd5-e758-7d7d-b360-514320231351" xlink:href="ck1437958-20260630.xsd#ck1437958_KingCountyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019edbd5-e758-7a73-b788-81cfbe327965" xlink:to="loc_ck1437958_KingCountyMember_019edbd5-e758-7d7d-b360-514320231351" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_IslandCountyMember_019edbd5-e758-70eb-977c-07ebe05577ed" xlink:href="ck1437958-20260630.xsd#ck1437958_IslandCountyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019edbd5-e758-7a73-b788-81cfbe327965" xlink:to="loc_ck1437958_IslandCountyMember_019edbd5-e758-70eb-977c-07ebe05577ed" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e758-7832-9498-29e6320185e6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019edbd5-e758-7ff6-b388-c471fe7c39c7" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e758-7832-9498-29e6320185e6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e758-7832-9498-29e6320185e6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e758-7832-9498-29e6320185e6" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e758-7832-9498-29e6320185e6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e758-75c2-9db6-17b4fc9426e9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e758-7832-9498-29e6320185e6" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e758-75c2-9db6-17b4fc9426e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXSegmentMember_019edbd5-e758-703f-8a1e-c2c3ff85985e" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e758-75c2-9db6-17b4fc9426e9" xlink:to="loc_ck1437958_CCBXSegmentMember_019edbd5-e758-703f-8a1e-c2c3ff85985e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_019edbd5-e758-7bd0-a21d-652c3a53692e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7583-9501-b44312979315" xlink:to="loc_us-gaap_NumberOfOperatingSegments_019edbd5-e758-7bd0-a21d-652c3a53692e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_019edbd5-e758-7ffc-b48d-110b118da941" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7583-9501-b44312979315" xlink:to="loc_us-gaap_NumberOfReportableSegments_019edbd5-e758-7ffc-b48d-110b118da941" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_NumberOfFullServiceBankingLocations_019edbd5-e758-787a-98c4-4aa48c002b43" xlink:href="ck1437958-20260630.xsd#ck1437958_NumberOfFullServiceBankingLocations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7583-9501-b44312979315" xlink:to="loc_ck1437958_NumberOfFullServiceBankingLocations_019edbd5-e758-787a-98c4-4aa48c002b43" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_NumberOfPartners_019edbd5-e758-72a3-9705-65428597a601" xlink:href="ck1437958-20260630.xsd#ck1437958_NumberOfPartners"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7583-9501-b44312979315" xlink:to="loc_ck1437958_NumberOfPartners_019edbd5-e758-72a3-9705-65428597a601" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/SegmentReportingSummaryofFinancialInformationofReportableSegmentsandReconciliationtoConsolidatedFinancialResultsDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#SegmentReportingSummaryofFinancialInformationofReportableSegmentsandReconciliationtoConsolidatedFinancialResultsDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/SegmentReportingSummaryofFinancialInformationofReportableSegmentsandReconciliationtoConsolidatedFinancialResultsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019edbd5-e758-7588-880e-7b47653b9685" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019edbd5-e758-7588-880e-7b47653b9685" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019edbd5-e758-7f43-948f-03d20b15d681" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019edbd5-e758-7588-880e-7b47653b9685" xlink:to="loc_srt_ConsolidationItemsAxis_019edbd5-e758-7f43-948f-03d20b15d681" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019edbd5-e758-7f43-948f-03d20b15d681_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019edbd5-e758-7f43-948f-03d20b15d681" xlink:to="loc_srt_ConsolidationItemsDomain_019edbd5-e758-7f43-948f-03d20b15d681_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019edbd5-e758-77ee-8a5b-e646e2f6925d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019edbd5-e758-7f43-948f-03d20b15d681" xlink:to="loc_srt_ConsolidationItemsDomain_019edbd5-e758-77ee-8a5b-e646e2f6925d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019edbd5-e758-75e6-8881-235af4cebaa1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019edbd5-e758-77ee-8a5b-e646e2f6925d" xlink:to="loc_us-gaap_OperatingSegmentsMember_019edbd5-e758-75e6-8881-235af4cebaa1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e758-7db9-8c5e-e7fb8678fe1a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019edbd5-e758-7588-880e-7b47653b9685" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e758-7db9-8c5e-e7fb8678fe1a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e758-7db9-8c5e-e7fb8678fe1a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e758-7db9-8c5e-e7fb8678fe1a" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e758-7db9-8c5e-e7fb8678fe1a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019edbd5-e758-7633-ad36-dc1a99a7745b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019edbd5-e758-7db9-8c5e-e7fb8678fe1a" xlink:to="loc_us-gaap_SegmentDomain_019edbd5-e758-7633-ad36-dc1a99a7745b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommunityBankSegmentMember_019edbd5-e758-7ec4-bbf3-d421b1f55e8c" xlink:href="ck1437958-20260630.xsd#ck1437958_CommunityBankSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e758-7633-ad36-dc1a99a7745b" xlink:to="loc_ck1437958_CommunityBankSegmentMember_019edbd5-e758-7ec4-bbf3-d421b1f55e8c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXSegmentMember_019edbd5-e758-735f-b290-b7deaa52a3e2" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e758-7633-ad36-dc1a99a7745b" xlink:to="loc_ck1437958_CCBXSegmentMember_019edbd5-e758-735f-b290-b7deaa52a3e2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_TreasureAndAdministrationMember_019edbd5-e758-7e3b-9288-2f1442e17c2c" xlink:href="ck1437958-20260630.xsd#ck1437958_TreasureAndAdministrationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019edbd5-e758-7633-ad36-dc1a99a7745b" xlink:to="loc_ck1437958_TreasureAndAdministrationMember_019edbd5-e758-7e3b-9288-2f1442e17c2c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019edbd5-e758-709e-ba71-7692f5cf920e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019edbd5-e758-7588-880e-7b47653b9685" xlink:to="loc_srt_ProductOrServiceAxis_019edbd5-e758-709e-ba71-7692f5cf920e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019edbd5-e758-709e-ba71-7692f5cf920e_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019edbd5-e758-709e-ba71-7692f5cf920e" xlink:to="loc_srt_ProductsAndServicesDomain_019edbd5-e758-709e-ba71-7692f5cf920e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019edbd5-e758-78c7-b57e-636cd1dbadb0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019edbd5-e758-709e-ba71-7692f5cf920e" xlink:to="loc_srt_ProductsAndServicesDomain_019edbd5-e758-78c7-b57e-636cd1dbadb0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_BaaSProgramIncomeMember_019edbd5-e758-7aed-971b-8ae403c1fcbb" xlink:href="ck1437958-20260630.xsd#ck1437958_BaaSProgramIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019edbd5-e758-78c7-b57e-636cd1dbadb0" xlink:to="loc_ck1437958_BaaSProgramIncomeMember_019edbd5-e758-7aed-971b-8ae403c1fcbb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_BaaSIndemnificationIncomeMember_019edbd5-e758-7aa3-a355-4c7c8fce20a0" xlink:href="ck1437958-20260630.xsd#ck1437958_BaaSIndemnificationIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019edbd5-e758-78c7-b57e-636cd1dbadb0" xlink:to="loc_ck1437958_BaaSIndemnificationIncomeMember_019edbd5-e758-7aa3-a355-4c7c8fce20a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_BaaSLoanExpenseMember_019edbd5-e758-7010-bdec-13a9881ea3ce" xlink:href="ck1437958-20260630.xsd#ck1437958_BaaSLoanExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019edbd5-e758-78c7-b57e-636cd1dbadb0" xlink:to="loc_ck1437958_BaaSLoanExpenseMember_019edbd5-e758-7010-bdec-13a9881ea3ce" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_BaaSFraudExpenseMember_019edbd5-e758-789f-9add-99dea675487b" xlink:href="ck1437958-20260630.xsd#ck1437958_BaaSFraudExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019edbd5-e758-78c7-b57e-636cd1dbadb0" xlink:to="loc_ck1437958_BaaSFraudExpenseMember_019edbd5-e758-789f-9add-99dea675487b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_019edbd5-e758-7ec8-b511-892d371435fc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_AssetsAbstract_019edbd5-e758-7ec8-b511-892d371435fc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CashAndDueFromBanksIncludingInterestBearingDepositsInBanks_019edbd5-e758-72e1-9065-abffc3645660" xlink:href="ck1437958-20260630.xsd#ck1437958_CashAndDueFromBanksIncludingInterestBearingDepositsInBanks"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019edbd5-e758-7ec8-b511-892d371435fc" xlink:to="loc_ck1437958_CashAndDueFromBanksIncludingInterestBearingDepositsInBanks_019edbd5-e758-72e1-9065-abffc3645660" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_IntrabankAssets_019edbd5-e758-7fbb-a575-f3c75fbcc7b5" xlink:href="ck1437958-20260630.xsd#ck1437958_IntrabankAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019edbd5-e758-7ec8-b511-892d371435fc" xlink:to="loc_ck1437958_IntrabankAssets_019edbd5-e758-7fbb-a575-f3c75fbcc7b5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecurities_019edbd5-e758-774c-b717-113f3152433e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019edbd5-e758-7ec8-b511-892d371435fc" xlink:to="loc_us-gaap_DebtSecurities_019edbd5-e758-774c-b717-113f3152433e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansReceivableHeldForSaleAmount_019edbd5-e758-7b50-aaba-c2993750d206" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansReceivableHeldForSaleAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019edbd5-e758-7ec8-b511-892d371435fc" xlink:to="loc_us-gaap_LoansReceivableHeldForSaleAmount_019edbd5-e758-7b50-aaba-c2993750d206" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e758-796f-86fb-bb85e9cb1a90" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019edbd5-e758-7ec8-b511-892d371435fc" xlink:to="loc_us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss_019edbd5-e758-796f-86fb-bb85e9cb1a90" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest_019edbd5-e758-7337-8887-4a4b087eb7c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019edbd5-e758-7ec8-b511-892d371435fc" xlink:to="loc_us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest_019edbd5-e758-7337-8887-4a4b087eb7c6" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_AllOtherAssets_019edbd5-e758-7b6c-849f-6c522a97b2b8" xlink:href="ck1437958-20260630.xsd#ck1437958_AllOtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019edbd5-e758-7ec8-b511-892d371435fc" xlink:to="loc_ck1437958_AllOtherAssets_019edbd5-e758-7b6c-849f-6c522a97b2b8" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019edbd5-e758-7c09-be84-20c286f88a13" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019edbd5-e758-7ec8-b511-892d371435fc" xlink:to="loc_us-gaap_Assets_019edbd5-e758-7c09-be84-20c286f88a13" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAbstract_019edbd5-e758-79af-b702-beaf6fe8e626" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_LiabilitiesAbstract_019edbd5-e758-79af-b702-beaf6fe8e626" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Deposits_019edbd5-e758-7b8f-a04d-a0afcd487282" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Deposits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019edbd5-e758-79af-b702-beaf6fe8e626" xlink:to="loc_us-gaap_Deposits_019edbd5-e758-7b8f-a04d-a0afcd487282" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherBorrowings_019edbd5-e758-7f7c-9156-729b75e79d78" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherBorrowings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019edbd5-e758-79af-b702-beaf6fe8e626" xlink:to="loc_us-gaap_OtherBorrowings_019edbd5-e758-7f7c-9156-729b75e79d78" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_IntrabankLiabilities_019edbd5-e758-7841-9df4-15ba5c98a39e" xlink:href="ck1437958-20260630.xsd#ck1437958_IntrabankLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019edbd5-e758-79af-b702-beaf6fe8e626" xlink:to="loc_ck1437958_IntrabankLiabilities_019edbd5-e758-7841-9df4-15ba5c98a39e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_AllOtherLiabilities_019edbd5-e758-76c1-acdf-d765d65a1a8c" xlink:href="ck1437958-20260630.xsd#ck1437958_AllOtherLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019edbd5-e758-79af-b702-beaf6fe8e626" xlink:to="loc_ck1437958_AllOtherLiabilities_019edbd5-e758-76c1-acdf-d765d65a1a8c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Liabilities_019edbd5-e758-7283-8210-bfc1ceb86f40" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Liabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019edbd5-e758-79af-b702-beaf6fe8e626" xlink:to="loc_us-gaap_Liabilities_019edbd5-e758-7283-8210-bfc1ceb86f40" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestAndDividendIncomeOperating_019edbd5-e758-702a-9e4f-8214a822824d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestAndDividendIncomeOperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_InterestAndDividendIncomeOperating_019edbd5-e758-702a-9e4f-8214a822824d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_InterestIncomeExpenseRelatedToTransferNet_019edbd5-e758-7c2c-87bd-da8d74cadff1" xlink:href="ck1437958-20260630.xsd#ck1437958_InterestIncomeExpenseRelatedToTransferNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_ck1437958_InterestIncomeExpenseRelatedToTransferNet_019edbd5-e758-7c2c-87bd-da8d74cadff1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_019edbd5-e758-7b53-9dc7-04ee1dfe4405" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_InterestExpense_019edbd5-e758-7b53-9dc7-04ee1dfe4405" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeExpenseNet_019edbd5-e758-77b7-af31-4d184aa02f3e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestIncomeExpenseNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_InterestIncomeExpenseNet_019edbd5-e758-77b7-af31-4d184aa02f3e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProvisionForLoanLeaseAndOtherLosses_019edbd5-e758-7aa6-a3d1-cc27d1cb0fc4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProvisionForLoanLeaseAndOtherLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_ProvisionForLoanLeaseAndOtherLosses_019edbd5-e758-7aa6-a3d1-cc27d1cb0fc4" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ProvisionRecaptureForUnfundedCommitments_019edbd5-e758-7970-b0f8-2abe08e4697c" xlink:href="ck1437958-20260630.xsd#ck1437958_ProvisionRecaptureForUnfundedCommitments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_ck1437958_ProvisionRecaptureForUnfundedCommitments_019edbd5-e758-7970-b0f8-2abe08e4697c" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeExpenseAfterProvisionForLoanLoss_019edbd5-e758-7196-bc38-7b534a9c441b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestIncomeExpenseAfterProvisionForLoanLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_InterestIncomeExpenseAfterProvisionForLoanLoss_019edbd5-e758-7196-bc38-7b534a9c441b" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_DepositServiceChargesAndFees_019edbd5-e758-7ed4-b36c-bb144bdfcee5" xlink:href="ck1437958-20260630.xsd#ck1437958_DepositServiceChargesAndFees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_ck1437958_DepositServiceChargesAndFees_019edbd5-e758-7ed4-b36c-bb144bdfcee5" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherIncome_019edbd5-e758-70fd-8973-9d52c6023251" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_OtherIncome_019edbd5-e758-70fd-8973-9d52c6023251" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_019edbd5-e758-7ab2-b84f-861c4ac49429" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_019edbd5-e758-7ab2-b84f-861c4ac49429" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoninterestIncome_019edbd5-e758-7430-9bff-cf0c4107291e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoninterestIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_NoninterestIncome_019edbd5-e758-7430-9bff-cf0c4107291e" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LaborAndRelatedExpense_019edbd5-e758-7590-aa19-26b2d625306e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LaborAndRelatedExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_LaborAndRelatedExpense_019edbd5-e758-7590-aa19-26b2d625306e" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OccupancyNet_019edbd5-e758-778b-9f99-c0104b319dd9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OccupancyNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_OccupancyNet_019edbd5-e758-778b-9f99-c0104b319dd9" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_SoftwareLicensesMaintenanceAndSubscriptions_019edbd5-e758-77e3-89a2-ffa4078e82de" xlink:href="ck1437958-20260630.xsd#ck1437958_SoftwareLicensesMaintenanceAndSubscriptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_ck1437958_SoftwareLicensesMaintenanceAndSubscriptions_019edbd5-e758-77e3-89a2-ffa4078e82de" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfessionalFees_019edbd5-e758-759d-a320-91891f501836" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProfessionalFees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_ProfessionalFees_019edbd5-e758-759d-a320-91891f501836" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CreditEnhancementReceivableValuationAdjustment_019fcdbb-d1da-7d8a-85fe-88978718ff70" xlink:href="ck1437958-20260630.xsd#ck1437958_CreditEnhancementReceivableValuationAdjustment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_ck1437958_CreditEnhancementReceivableValuationAdjustment_019fcdbb-d1da-7d8a-85fe-88978718ff70" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherExpenses_019edbd5-e758-7f10-90fc-e4e2af53a0e2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_OtherExpenses_019edbd5-e758-7f10-90fc-e4e2af53a0e2" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_BankingServiceExpenses_019edbd5-e758-788f-bd42-ffc27713793c" xlink:href="ck1437958-20260630.xsd#ck1437958_BankingServiceExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_ck1437958_BankingServiceExpenses_019edbd5-e758-788f-bd42-ffc27713793c" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoninterestExpense_019edbd5-e758-7055-b5dc-bd01171a5e2f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoninterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_NoninterestExpense_019edbd5-e758-7055-b5dc-bd01171a5e2f" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019edbd5-e758-77b7-91bb-504eac639c76" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019edbd5-e758-77b7-91bb-504eac639c76" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019edbd5-e758-7a35-a874-84a4e8f6f322" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019edbd5-e758-7a35-a874-84a4e8f6f322" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019edbd5-e758-7c4e-9f01-0358723e5d5a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019edbd5-e758-7f03-86bf-8c3597ccf1ea" xlink:to="loc_us-gaap_NetIncomeLoss_019edbd5-e758-7c4e-9f01-0358723e5d5a" xlink:type="arc" order="23"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/CommitmentsandContingenciesAdditionalInformationDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#CommitmentsandContingenciesAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/CommitmentsandContingenciesAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_019fcdc4-2062-72c3-9a9d-eadffaa76e23" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_019fcdc4-2062-7740-8043-e35d4dedcde5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_019fcdc4-2062-72c3-9a9d-eadffaa76e23" xlink:to="loc_us-gaap_LossContingenciesTable_019fcdc4-2062-7740-8043-e35d4dedcde5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019fcdc4-2062-716a-a5a6-1b9c4fd13f8e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019fcdc4-2062-7740-8043-e35d4dedcde5" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019fcdc4-2062-716a-a5a6-1b9c4fd13f8e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019fcdc4-2062-716a-a5a6-1b9c4fd13f8e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019fcdc4-2062-716a-a5a6-1b9c4fd13f8e" xlink:to="loc_us-gaap_SegmentDomain_019fcdc4-2062-716a-a5a6-1b9c4fd13f8e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019fcdc4-2062-7c54-8828-3f229fdc6308" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019fcdc4-2062-716a-a5a6-1b9c4fd13f8e" xlink:to="loc_us-gaap_SegmentDomain_019fcdc4-2062-7c54-8828-3f229fdc6308" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXSegmentMember_019fcdc4-2062-74d5-b3fb-11bcb9463202" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019fcdc4-2062-7c54-8828-3f229fdc6308" xlink:to="loc_ck1437958_CCBXSegmentMember_019fcdc4-2062-74d5-b3fb-11bcb9463202" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_019fcdcc-9e43-77ef-9bc7-4882b58b780d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesByNatureOfContingencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019fcdc4-2062-7740-8043-e35d4dedcde5" xlink:to="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_019fcdcc-9e43-77ef-9bc7-4882b58b780d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyNatureDomain_019fcdcc-9e43-77ef-9bc7-4882b58b780d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingencyNatureDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_019fcdcc-9e43-77ef-9bc7-4882b58b780d" xlink:to="loc_us-gaap_LossContingencyNatureDomain_019fcdcc-9e43-77ef-9bc7-4882b58b780d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyNatureDomain_019fcdcc-9e43-7f40-9ab1-2080d9f9545d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingencyNatureDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_019fcdcc-9e43-77ef-9bc7-4882b58b780d" xlink:to="loc_us-gaap_LossContingencyNatureDomain_019fcdcc-9e43-7f40-9ab1-2080d9f9545d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommitmentsToExtendCreditMember_019fcdcc-9e43-73aa-a6b8-44d09fcca9de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommitmentsToExtendCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingencyNatureDomain_019fcdcc-9e43-7f40-9ab1-2080d9f9545d" xlink:to="loc_us-gaap_CommitmentsToExtendCreditMember_019fcdcc-9e43-73aa-a6b8-44d09fcca9de" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXSoldReceivableCommitmentMember_019fcdce-5b28-7719-aa35-4cf7eab772d9" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXSoldReceivableCommitmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingencyNatureDomain_019fcdcc-9e43-7f40-9ab1-2080d9f9545d" xlink:to="loc_ck1437958_CCBXSoldReceivableCommitmentMember_019fcdce-5b28-7719-aa35-4cf7eab772d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralAxis_019fcdcf-d064-70a8-9460-e144a4946b99" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019fcdc4-2062-7740-8043-e35d4dedcde5" xlink:to="loc_us-gaap_CollateralAxis_019fcdcf-d064-70a8-9460-e144a4946b99" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralDomain_019fcdcf-d064-70a8-9460-e144a4946b99_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CollateralAxis_019fcdcf-d064-70a8-9460-e144a4946b99" xlink:to="loc_us-gaap_CollateralDomain_019fcdcf-d064-70a8-9460-e144a4946b99_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralDomain_019fcdcf-d064-7c3b-8de0-4e57ab4f71b5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CollateralAxis_019fcdcf-d064-70a8-9460-e144a4946b99" xlink:to="loc_us-gaap_CollateralDomain_019fcdcf-d064-7c3b-8de0-4e57ab4f71b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CapitalCallLinesMember_019fcddc-aaa3-75b1-a380-b753f3e15e43" xlink:href="ck1437958-20260630.xsd#ck1437958_CapitalCallLinesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019fcdcf-d064-7c3b-8de0-4e57ab4f71b5" xlink:to="loc_ck1437958_CapitalCallLinesMember_019fcddc-aaa3-75b1-a380-b753f3e15e43" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019fcdcf-d064-7b50-b234-e5a121d2f502" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019fcdcf-d064-7c3b-8de0-4e57ab4f71b5" xlink:to="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019fcdcf-d064-7b50-b234-e5a121d2f502" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019fcdd6-ee07-738c-9fc6-46903290dd65" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019fcdc4-2062-7740-8043-e35d4dedcde5" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019fcdd6-ee07-738c-9fc6-46903290dd65" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019fcdd6-ee07-738c-9fc6-46903290dd65_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019fcdd6-ee07-738c-9fc6-46903290dd65" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019fcdd6-ee07-738c-9fc6-46903290dd65_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019fcdd6-ee07-7d75-9782-8c85239afc59" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis_019fcdd6-ee07-738c-9fc6-46903290dd65" xlink:to="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019fcdd6-ee07-7d75-9782-8c85239afc59" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditCardMember_019fcdd7-0486-7687-94b9-73e4ae317bbf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditCardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019fcdd6-ee07-7d75-9782-8c85239afc59" xlink:to="loc_us-gaap_CreditCardMember_019fcdd7-0486-7687-94b9-73e4ae317bbf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialRealEstateMember_019fcdd7-6aac-7ea2-ab4b-30088a6ee1f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResidentialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019fcdd6-ee07-7d75-9782-8c85239afc59" xlink:to="loc_us-gaap_ResidentialRealEstateMember_019fcdd7-6aac-7ea2-ab4b-30088a6ee1f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConsumerAndOtherLoansMember_019fcdd7-e53f-769f-8d13-0c82b0e2952a" xlink:href="ck1437958-20260630.xsd#ck1437958_ConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019fcdd6-ee07-7d75-9782-8c85239afc59" xlink:to="loc_ck1437958_ConsumerAndOtherLoansMember_019fcdd7-e53f-769f-8d13-0c82b0e2952a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialAndIndustrialCapitalCallLineCommitmentsMember_019fcdd8-838c-79f6-8403-5139adf3b0b6" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialAndIndustrialCapitalCallLineCommitmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019fcdd6-ee07-7d75-9782-8c85239afc59" xlink:to="loc_ck1437958_CommercialAndIndustrialCapitalCallLineCommitmentsMember_019fcdd8-838c-79f6-8403-5139adf3b0b6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialConstructionLoansMember_019fcdd9-3699-7a86-86f8-804e1fb054b3" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialConstructionLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancingReceivableRecordedInvestmentClassOfFinancingReceivableDomain_019fcdd6-ee07-7d75-9782-8c85239afc59" xlink:to="loc_ck1437958_CommercialConstructionLoansMember_019fcdd9-3699-7a86-86f8-804e1fb054b3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019fcdd9-ef4e-7ffa-b0b7-2b080d0d93e4" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019fcdc4-2062-7740-8043-e35d4dedcde5" xlink:to="loc_srt_RangeAxis_019fcdd9-ef4e-7ffa-b0b7-2b080d0d93e4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019fcdd9-ef4e-7ffa-b0b7-2b080d0d93e4_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019fcdd9-ef4e-7ffa-b0b7-2b080d0d93e4" xlink:to="loc_srt_RangeMember_019fcdd9-ef4e-7ffa-b0b7-2b080d0d93e4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019fcdd9-ef4e-7ad5-8620-a278780f2b12" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019fcdd9-ef4e-7ffa-b0b7-2b080d0d93e4" xlink:to="loc_srt_RangeMember_019fcdd9-ef4e-7ad5-8620-a278780f2b12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019fcdda-3836-79e9-99df-3bd772329ac9" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019fcdd9-ef4e-7ad5-8620-a278780f2b12" xlink:to="loc_srt_MaximumMember_019fcdda-3836-79e9-99df-3bd772329ac9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnusedCommitmentsToExtendCredit_019fcdc4-2062-7a50-abe4-06d819e6c55d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnusedCommitmentsToExtendCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019fcdc4-2062-72c3-9a9d-eadffaa76e23" xlink:to="loc_us-gaap_UnusedCommitmentsToExtendCredit_019fcdc4-2062-7a50-abe4-06d819e6c55d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_IncreaseDecreaseInUnusedCommitmentsToExtendCredit_019fcdd5-99b8-71c6-88b5-a10aca82cb8d" xlink:href="ck1437958-20260630.xsd#ck1437958_IncreaseDecreaseInUnusedCommitmentsToExtendCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019fcdc4-2062-72c3-9a9d-eadffaa76e23" xlink:to="loc_ck1437958_IncreaseDecreaseInUnusedCommitmentsToExtendCredit_019fcdd5-99b8-71c6-88b5-a10aca82cb8d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_UnusedCommitmentsToExtendCreditCanceledUnconditionally_019fcdc4-2062-75d0-9534-e3fb6facc2fa" xlink:href="ck1437958-20260630.xsd#ck1437958_UnusedCommitmentsToExtendCreditCanceledUnconditionally"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019fcdc4-2062-72c3-9a9d-eadffaa76e23" xlink:to="loc_ck1437958_UnusedCommitmentsToExtendCreditCanceledUnconditionally_019fcdc4-2062-75d0-9534-e3fb6facc2fa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyLossInPeriod_019fcdc4-2062-7a72-9dab-1f0977255734" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingencyLossInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019fcdc4-2062-72c3-9a9d-eadffaa76e23" xlink:to="loc_us-gaap_LossContingencyLossInPeriod_019fcdc4-2062-7a72-9dab-1f0977255734" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/CommitmentsandContingenciesScheduleofFinancialInstrumentsContractAmountRepresentsCreditRiskDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#CommitmentsandContingenciesScheduleofFinancialInstrumentsContractAmountRepresentsCreditRiskDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/CommitmentsandContingenciesScheduleofFinancialInstrumentsContractAmountRepresentsCreditRiskDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_019fcdc4-2062-74ca-a6f7-799946a08684" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_019fcdc4-2062-79f4-b6ec-1ce8586d4d88" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_019fcdc4-2062-74ca-a6f7-799946a08684" xlink:to="loc_us-gaap_LossContingenciesTable_019fcdc4-2062-79f4-b6ec-1ce8586d4d88" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesByIndustryAxis_019fcdc4-2062-7b2b-a0dd-0e50a926ba51" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesByIndustryAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019fcdc4-2062-79f4-b6ec-1ce8586d4d88" xlink:to="loc_us-gaap_EquitySecuritiesByIndustryAxis_019fcdc4-2062-7b2b-a0dd-0e50a926ba51" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesIndustryMember_019fcdc4-2062-7b2b-a0dd-0e50a926ba51_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EquitySecuritiesByIndustryAxis_019fcdc4-2062-7b2b-a0dd-0e50a926ba51" xlink:to="loc_us-gaap_EquitySecuritiesIndustryMember_019fcdc4-2062-7b2b-a0dd-0e50a926ba51_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesIndustryMember_019fcdc4-2062-72d6-b230-db672dd6888e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EquitySecuritiesByIndustryAxis_019fcdc4-2062-7b2b-a0dd-0e50a926ba51" xlink:to="loc_us-gaap_EquitySecuritiesIndustryMember_019fcdc4-2062-72d6-b230-db672dd6888e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialAndIndustrialSectorMember_019fcdc4-2062-7388-a90a-353fb874dade" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialAndIndustrialSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019fcdc4-2062-72d6-b230-db672dd6888e" xlink:to="loc_us-gaap_CommercialAndIndustrialSectorMember_019fcdc4-2062-7388-a90a-353fb874dade" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralAxis_019fcdc4-2062-7e6a-b1e8-24a690491e07" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019fcdc4-2062-79f4-b6ec-1ce8586d4d88" xlink:to="loc_us-gaap_CollateralAxis_019fcdc4-2062-7e6a-b1e8-24a690491e07" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralDomain_019fcdc4-2062-7e6a-b1e8-24a690491e07_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CollateralAxis_019fcdc4-2062-7e6a-b1e8-24a690491e07" xlink:to="loc_us-gaap_CollateralDomain_019fcdc4-2062-7e6a-b1e8-24a690491e07_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralDomain_019fcdc4-2062-7ea5-a772-8d461d544236" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CollateralAxis_019fcdc4-2062-7e6a-b1e8-24a690491e07" xlink:to="loc_us-gaap_CollateralDomain_019fcdc4-2062-7ea5-a772-8d461d544236" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019fcdc4-2062-75da-a01b-3c2265aa3799" xlink:href="ck1437958-20260630.xsd#ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019fcdc4-2062-7ea5-a772-8d461d544236" xlink:to="loc_ck1437958_CommercialAndIndustrialLoansPortfolioSegmentMember_019fcdc4-2062-75da-a01b-3c2265aa3799" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConstructionCommercialRealEstateMember_019fcdc4-2062-7bd0-a779-7aa93c50434f" xlink:href="ck1437958-20260630.xsd#ck1437958_ConstructionCommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019fcdc4-2062-7ea5-a772-8d461d544236" xlink:to="loc_ck1437958_ConstructionCommercialRealEstateMember_019fcdc4-2062-7bd0-a779-7aa93c50434f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConstructionResidentialRealEstateMember_019fcdc4-2062-7f05-bcd5-5a2e166a96d5" xlink:href="ck1437958-20260630.xsd#ck1437958_ConstructionResidentialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019fcdc4-2062-7ea5-a772-8d461d544236" xlink:to="loc_ck1437958_ConstructionResidentialRealEstateMember_019fcdc4-2062-7f05-bcd5-5a2e166a96d5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialRealEstateMember_019fcdc4-2062-7178-89e0-4293139397d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResidentialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019fcdc4-2062-7ea5-a772-8d461d544236" xlink:to="loc_us-gaap_ResidentialRealEstateMember_019fcdc4-2062-7178-89e0-4293139397d3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialRealEstateMember_019fcdc4-2062-7c32-9391-fd76db06c57b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019fcdc4-2062-7ea5-a772-8d461d544236" xlink:to="loc_us-gaap_CommercialRealEstateMember_019fcdc4-2062-7c32-9391-fd76db06c57b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditCardMember_019fcdc4-2062-7162-a35a-c73478ceb77b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditCardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019fcdc4-2062-7ea5-a772-8d461d544236" xlink:to="loc_us-gaap_CreditCardMember_019fcdc4-2062-7162-a35a-c73478ceb77b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConsumerAndOtherLoansMember_019fcdc4-2062-777a-91da-8a28e52ab4db" xlink:href="ck1437958-20260630.xsd#ck1437958_ConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019fcdc4-2062-7ea5-a772-8d461d544236" xlink:to="loc_ck1437958_ConsumerAndOtherLoansMember_019fcdc4-2062-777a-91da-8a28e52ab4db" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_019fcdc4-2062-74cc-820f-0a843a12c3cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesByNatureOfContingencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019fcdc4-2062-79f4-b6ec-1ce8586d4d88" xlink:to="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_019fcdc4-2062-74cc-820f-0a843a12c3cb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyNatureDomain_019fcdc4-2062-74cc-820f-0a843a12c3cb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingencyNatureDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_019fcdc4-2062-74cc-820f-0a843a12c3cb" xlink:to="loc_us-gaap_LossContingencyNatureDomain_019fcdc4-2062-74cc-820f-0a843a12c3cb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyNatureDomain_019fcdc4-2062-7a1d-b25d-d90779dbf979" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingencyNatureDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_019fcdc4-2062-74cc-820f-0a843a12c3cb" xlink:to="loc_us-gaap_LossContingencyNatureDomain_019fcdc4-2062-7a1d-b25d-d90779dbf979" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommitmentsToExtendCreditMember_019fcdc4-2062-7540-9a1d-0fdb5011dff4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommitmentsToExtendCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingencyNatureDomain_019fcdc4-2062-7a1d-b25d-d90779dbf979" xlink:to="loc_us-gaap_CommitmentsToExtendCreditMember_019fcdc4-2062-7540-9a1d-0fdb5011dff4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXSoldReceivableCommitmentMember_019fcdc7-776e-72cf-a1c5-3129b7e8d62f" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXSoldReceivableCommitmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CommitmentsToExtendCreditMember_019fcdc4-2062-7540-9a1d-0fdb5011dff4" xlink:to="loc_ck1437958_CCBXSoldReceivableCommitmentMember_019fcdc7-776e-72cf-a1c5-3129b7e8d62f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StandbyLettersOfCreditMember_019fcdc4-2062-78b3-b101-e3437712fc6a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StandbyLettersOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CommitmentsToExtendCreditMember_019fcdc4-2062-7540-9a1d-0fdb5011dff4" xlink:to="loc_us-gaap_StandbyLettersOfCreditMember_019fcdc4-2062-78b3-b101-e3437712fc6a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_EquityInvestmentCommitmentsMember_019fcdc4-2062-7320-a5a1-8464c0853717" xlink:href="ck1437958-20260630.xsd#ck1437958_EquityInvestmentCommitmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CommitmentsToExtendCreditMember_019fcdc4-2062-7540-9a1d-0fdb5011dff4" xlink:to="loc_ck1437958_EquityInvestmentCommitmentsMember_019fcdc4-2062-7320-a5a1-8464c0853717" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnusedCommitmentsToExtendCredit_019fcdc4-2062-75a8-be29-791b59d0acd1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnusedCommitmentsToExtendCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019fcdc4-2062-74ca-a6f7-799946a08684" xlink:to="loc_us-gaap_UnusedCommitmentsToExtendCredit_019fcdc4-2062-75a8-be29-791b59d0acd1" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/CommitmentsandContingenciesSummaryofCommitmentstoExtendCreditonCCBXLoansDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#CommitmentsandContingenciesSummaryofCommitmentstoExtendCreditonCCBXLoansDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/CommitmentsandContingenciesSummaryofCommitmentstoExtendCreditonCCBXLoansDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_019fcdc4-2062-71d1-8ade-ce5e3097f8fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_019fcdc4-2062-7468-a3e4-a409c15960a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_019fcdc4-2062-71d1-8ade-ce5e3097f8fd" xlink:to="loc_us-gaap_LossContingenciesTable_019fcdc4-2062-7468-a3e4-a409c15960a5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesByIndustryAxis_019fcdc4-2062-743b-8cfa-bf8915d68590" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesByIndustryAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019fcdc4-2062-7468-a3e4-a409c15960a5" xlink:to="loc_us-gaap_EquitySecuritiesByIndustryAxis_019fcdc4-2062-743b-8cfa-bf8915d68590" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesIndustryMember_019fcdc4-2062-743b-8cfa-bf8915d68590_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EquitySecuritiesByIndustryAxis_019fcdc4-2062-743b-8cfa-bf8915d68590" xlink:to="loc_us-gaap_EquitySecuritiesIndustryMember_019fcdc4-2062-743b-8cfa-bf8915d68590_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesIndustryMember_019fcdc4-2062-7db4-93af-cb62eb7afa29" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EquitySecuritiesByIndustryAxis_019fcdc4-2062-743b-8cfa-bf8915d68590" xlink:to="loc_us-gaap_EquitySecuritiesIndustryMember_019fcdc4-2062-7db4-93af-cb62eb7afa29" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialAndIndustrialSectorMember_019fcdc4-2062-7043-bf7e-a22f04412570" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialAndIndustrialSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019fcdc4-2062-7db4-93af-cb62eb7afa29" xlink:to="loc_us-gaap_CommercialAndIndustrialSectorMember_019fcdc4-2062-7043-bf7e-a22f04412570" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralAxis_019fcdc4-2062-7e3f-b279-e9668785e630" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019fcdc4-2062-7468-a3e4-a409c15960a5" xlink:to="loc_us-gaap_CollateralAxis_019fcdc4-2062-7e3f-b279-e9668785e630" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralDomain_019fcdc4-2062-7e3f-b279-e9668785e630_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CollateralAxis_019fcdc4-2062-7e3f-b279-e9668785e630" xlink:to="loc_us-gaap_CollateralDomain_019fcdc4-2062-7e3f-b279-e9668785e630_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralDomain_019fcdc4-2062-72b0-9368-5ae921bb7130" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CollateralAxis_019fcdc4-2062-7e3f-b279-e9668785e630" xlink:to="loc_us-gaap_CollateralDomain_019fcdc4-2062-72b0-9368-5ae921bb7130" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXCapitalCallLinesMember_019fcdc4-2062-7616-b625-08b5286da44f" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXCapitalCallLinesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019fcdc4-2062-72b0-9368-5ae921bb7130" xlink:to="loc_ck1437958_CCBXCapitalCallLinesMember_019fcdc4-2062-7616-b625-08b5286da44f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialRealEstateMember_019fcdc4-2062-7510-9aa3-5440fa371867" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResidentialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019fcdc4-2062-72b0-9368-5ae921bb7130" xlink:to="loc_us-gaap_ResidentialRealEstateMember_019fcdc4-2062-7510-9aa3-5440fa371867" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConsumerAndOtherLoansMember_019fcdc4-2062-713e-85b4-2c5659cdac4d" xlink:href="ck1437958-20260630.xsd#ck1437958_ConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019fcdc4-2062-72b0-9368-5ae921bb7130" xlink:to="loc_ck1437958_ConsumerAndOtherLoansMember_019fcdc4-2062-713e-85b4-2c5659cdac4d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXSoldReceivableCommitmentMember_019fcdca-1b2c-7b6b-ad5b-fb98b60a69dd" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXSoldReceivableCommitmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019fcdc4-2062-72b0-9368-5ae921bb7130" xlink:to="loc_ck1437958_CCBXSoldReceivableCommitmentMember_019fcdca-1b2c-7b6b-ad5b-fb98b60a69dd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019fcdc4-2062-70ff-9253-81fd22748bf4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019fcdc4-2062-7468-a3e4-a409c15960a5" xlink:to="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019fcdc4-2062-70ff-9253-81fd22748bf4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019fcdc4-2062-70ff-9253-81fd22748bf4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019fcdc4-2062-70ff-9253-81fd22748bf4" xlink:to="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019fcdc4-2062-70ff-9253-81fd22748bf4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019fcdc4-2062-7c8d-b53f-3adebdd8c964" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019fcdc4-2062-70ff-9253-81fd22748bf4" xlink:to="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019fcdc4-2062-7c8d-b53f-3adebdd8c964" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXLoansMember_019fcdc4-2062-706e-ac57-e9373b65040b" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019fcdc4-2062-7c8d-b53f-3adebdd8c964" xlink:to="loc_ck1437958_CCBXLoansMember_019fcdc4-2062-706e-ac57-e9373b65040b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnusedCommitmentsToExtendCredit_019fcdc4-2062-7286-9be3-94fd95c55837" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnusedCommitmentsToExtendCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019fcdc4-2062-71d1-8ade-ce5e3097f8fd" xlink:to="loc_us-gaap_UnusedCommitmentsToExtendCredit_019fcdc4-2062-7286-9be3-94fd95c55837" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.coastalbank.com/role/CommitmentsandContingenciesScheduleofCCBXPartnersMaximumAggregateCustomerLoanBalancesDetails" xlink:type="simple" xlink:href="ck1437958-20260630.xsd#CommitmentsandContingenciesScheduleofCCBXPartnersMaximumAggregateCustomerLoanBalancesDetails"/>
  <link:definitionLink xlink:role="http://www.coastalbank.com/role/CommitmentsandContingenciesScheduleofCCBXPartnersMaximumAggregateCustomerLoanBalancesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsLineItems_019fcdc4-2062-7e4c-9881-1011cfdc83c5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherCommitmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsTable_019fcdc4-2062-7f3c-828e-b1cc5039b8d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherCommitmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019fcdc4-2062-7e4c-9881-1011cfdc83c5" xlink:to="loc_us-gaap_OtherCommitmentsTable_019fcdc4-2062-7f3c-828e-b1cc5039b8d2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesByIndustryAxis_019fcdc4-2062-7333-86ed-5c0415e8d3a6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesByIndustryAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_019fcdc4-2062-7f3c-828e-b1cc5039b8d2" xlink:to="loc_us-gaap_EquitySecuritiesByIndustryAxis_019fcdc4-2062-7333-86ed-5c0415e8d3a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesIndustryMember_019fcdc4-2062-7333-86ed-5c0415e8d3a6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EquitySecuritiesByIndustryAxis_019fcdc4-2062-7333-86ed-5c0415e8d3a6" xlink:to="loc_us-gaap_EquitySecuritiesIndustryMember_019fcdc4-2062-7333-86ed-5c0415e8d3a6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesIndustryMember_019fcdc4-2062-774a-9434-3a3a8ee6fe70" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EquitySecuritiesByIndustryAxis_019fcdc4-2062-7333-86ed-5c0415e8d3a6" xlink:to="loc_us-gaap_EquitySecuritiesIndustryMember_019fcdc4-2062-774a-9434-3a3a8ee6fe70" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialAndIndustrialSectorMember_019fcdc4-2062-767f-867b-70f9f56a89e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialAndIndustrialSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019fcdc4-2062-774a-9434-3a3a8ee6fe70" xlink:to="loc_us-gaap_CommercialAndIndustrialSectorMember_019fcdc4-2062-767f-867b-70f9f56a89e1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CapitalCallLinesMember_019fcdc4-2062-73a4-81f5-1bad3ecda9c7" xlink:href="ck1437958-20260630.xsd#ck1437958_CapitalCallLinesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CommercialAndIndustrialSectorMember_019fcdc4-2062-767f-867b-70f9f56a89e1" xlink:to="loc_ck1437958_CapitalCallLinesMember_019fcdc4-2062-73a4-81f5-1bad3ecda9c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_AllOtherCommercialIndustrialLoansMember_019fcdc4-2062-755c-8284-9e4eeb4c9951" xlink:href="ck1437958-20260630.xsd#ck1437958_AllOtherCommercialIndustrialLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CommercialAndIndustrialSectorMember_019fcdc4-2062-767f-867b-70f9f56a89e1" xlink:to="loc_ck1437958_AllOtherCommercialIndustrialLoansMember_019fcdc4-2062-755c-8284-9e4eeb4c9951" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralAxis_019fcdc4-2062-7c3e-9ff3-64b387bb804b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_019fcdc4-2062-7f3c-828e-b1cc5039b8d2" xlink:to="loc_us-gaap_CollateralAxis_019fcdc4-2062-7c3e-9ff3-64b387bb804b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralDomain_019fcdc4-2062-7c3e-9ff3-64b387bb804b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CollateralAxis_019fcdc4-2062-7c3e-9ff3-64b387bb804b" xlink:to="loc_us-gaap_CollateralDomain_019fcdc4-2062-7c3e-9ff3-64b387bb804b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollateralDomain_019fcdc4-2062-7700-9415-56983aaef91b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollateralDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CollateralAxis_019fcdc4-2062-7c3e-9ff3-64b387bb804b" xlink:to="loc_us-gaap_CollateralDomain_019fcdc4-2062-7700-9415-56983aaef91b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResidentialRealEstateMember_019fcdc4-2062-7a79-b2c0-4d3df8663c84" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResidentialRealEstateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019fcdc4-2062-7700-9415-56983aaef91b" xlink:to="loc_us-gaap_ResidentialRealEstateMember_019fcdc4-2062-7a79-b2c0-4d3df8663c84" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_HomeEquityLinesOfCreditMember_019fcdc4-2062-7d0c-8134-998ba7c0a8e0" xlink:href="ck1437958-20260630.xsd#ck1437958_HomeEquityLinesOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResidentialRealEstateMember_019fcdc4-2062-7a79-b2c0-4d3df8663c84" xlink:to="loc_ck1437958_HomeEquityLinesOfCreditMember_019fcdc4-2062-7d0c-8134-998ba7c0a8e0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_ConsumerAndOtherLoansMember_019fcdc4-2062-7a46-9269-1c567e7dc0ce" xlink:href="ck1437958-20260630.xsd#ck1437958_ConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollateralDomain_019fcdc4-2062-7700-9415-56983aaef91b" xlink:to="loc_ck1437958_ConsumerAndOtherLoansMember_019fcdc4-2062-7a46-9269-1c567e7dc0ce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CreditCardsMember_019fcdc4-2062-756f-bf58-1d0316dfff15" xlink:href="ck1437958-20260630.xsd#ck1437958_CreditCardsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ck1437958_ConsumerAndOtherLoansMember_019fcdc4-2062-7a46-9269-1c567e7dc0ce" xlink:to="loc_ck1437958_CreditCardsMember_019fcdc4-2062-756f-bf58-1d0316dfff15" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_InstallmentLoansMember_019fcdc4-2062-77ec-ab35-53746d42d22c" xlink:href="ck1437958-20260630.xsd#ck1437958_InstallmentLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ck1437958_ConsumerAndOtherLoansMember_019fcdc4-2062-7a46-9269-1c567e7dc0ce" xlink:to="loc_ck1437958_InstallmentLoansMember_019fcdc4-2062-77ec-ab35-53746d42d22c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_OtherConsumerAndOtherLoansMember_019fcdc4-2062-7ce1-bb54-dfd523e1ba4d" xlink:href="ck1437958-20260630.xsd#ck1437958_OtherConsumerAndOtherLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ck1437958_ConsumerAndOtherLoansMember_019fcdc4-2062-7a46-9269-1c567e7dc0ce" xlink:to="loc_ck1437958_OtherConsumerAndOtherLoansMember_019fcdc4-2062-7ce1-bb54-dfd523e1ba4d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019fcdc4-2062-7be2-be66-559aa9de5c80" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_019fcdc4-2062-7f3c-828e-b1cc5039b8d2" xlink:to="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019fcdc4-2062-7be2-be66-559aa9de5c80" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019fcdc4-2062-7be2-be66-559aa9de5c80_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019fcdc4-2062-7be2-be66-559aa9de5c80" xlink:to="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019fcdc4-2062-7be2-be66-559aa9de5c80_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019fcdc4-2062-7784-bbee-0d0255b23f91" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesAxis_019fcdc4-2062-7be2-be66-559aa9de5c80" xlink:to="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019fcdc4-2062-7784-bbee-0d0255b23f91" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_CCBXLoansMember_019fcdc4-2062-72b3-90c3-6852c1033206" xlink:href="ck1437958-20260630.xsd#ck1437958_CCBXLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LoansInsuredOrGuaranteedByGovernmentAuthoritiesDomain_019fcdc4-2062-7784-bbee-0d0255b23f91" xlink:to="loc_ck1437958_CCBXLoansMember_019fcdc4-2062-72b3-90c3-6852c1033206" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_MaximumAggregateCustomerLoanBalanceAmount_019fcdc4-2062-7d92-95cf-1a5fbb176d19" xlink:href="ck1437958-20260630.xsd#ck1437958_MaximumAggregateCustomerLoanBalanceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019fcdc4-2062-7e4c-9881-1011cfdc83c5" xlink:to="loc_ck1437958_MaximumAggregateCustomerLoanBalanceAmount_019fcdc4-2062-7d92-95cf-1a5fbb176d19" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#IndividualsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllIndividualsMember" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#ecd_AllIndividualsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck1437958_BrianHamiltonMember_019fd495-93e7-7131-9d43-b5654368f7c4" xlink:href="ck1437958-20260630.xsd#ck1437958_BrianHamiltonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_ck1437958_BrianHamiltonMember_019fd495-93e7-7131-9d43-b5654368f7c4" xlink:type="arc" order="2"/>
  </link:definitionLink>
</link:linkbase>
