<?xml version="1.0" encoding="utf-8"?>
<!--Generated by DFIN XBRL Instance Document - http://www.dfinsolutions.com/ - Version R3.0 - on 12-May-2026 [01:28:34] {AM}-->
<schema targetNamespace="http://www.SixthStreetSpecialtyLending.com/20260507" elementFormDefault="qualified" xmlns:tslx="http://www.SixthStreetSpecialtyLending.com/20260507" xmlns="http://www.w3.org/2001/XMLSchema" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xl="http://www.xbrl.org/2003/XLink" xmlns:us-gaap="http://fasb.org/us-gaap/2025" xmlns:country="http://xbrl.sec.gov/country/2025" xmlns:currency="http://xbrl.sec.gov/currency/2025" xmlns:dei="http://xbrl.sec.gov/dei/2025" xmlns:exch="http://xbrl.sec.gov/exch/2025" xmlns:naics="http://xbrl.sec.gov/naics/2025" xmlns:sic="http://xbrl.sec.gov/sic/2025" xmlns:us-types="http://fasb.org/us-types/2025" xmlns:stpr="http://xbrl.sec.gov/stpr/2025" xmlns:deprecated="http://www.xbrl.org/2009/arcrole/fact-explanatoryFact" xmlns:srt="http://fasb.org/srt/2025" xmlns:srt-types="http://fasb.org/srt-types/2025" xmlns:dtr-types="http://www.xbrl.org/dtr/type/2024-01-31" xmlns:cef="http://xbrl.sec.gov/cef/2025" xmlns:cef-pre="http://xbrl.sec.gov/cef-pre/2025">
  <annotation>
    <appinfo>
      <link:roleType roleURI="http://www.SixthStreetSpecialtyLending.com/role/N2SeniorSecurities" id="N2SeniorSecurities">
        <link:definition>1001 - Disclosure - N-2 - [Senior Securities]</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.SixthStreetSpecialtyLending.com/role/N2RiskFactors" id="N2RiskFactors">
        <link:definition>1002 - Disclosure - N-2 - [Risk Factors]</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://www.SixthStreetSpecialtyLending.com/role/N2LongTermDebt" id="N2LongTermDebt">
        <link:definition>1003 - Disclosure - N-2 - [Long Term Debt]</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
      </link:roleType>
      <link:linkbase id="lnk" >
        <link:roleRef xlink:type="simple" xlink:href="https://xbrl.sec.gov/cef/2025/cef-2025.xsd#N2" roleURI="http://xbrl.sec.gov/cef/role/N2"  />
        <link:roleRef xlink:type="simple" xlink:href="https://xbrl.sec.gov/cef/2025/cef-2025.xsd#RiskOnly" roleURI="http://xbrl.sec.gov/cef/role/RiskOnly"  />
        <link:roleRef xlink:type="simple" xlink:href="https://xbrl.sec.gov/cef/2025/cef-2025.xsd#SecurityOnly" roleURI="http://xbrl.sec.gov/cef/role/SecurityOnly"  />
        <link:roleRef xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/role/negated-2009-12-16.xsd#negatedLabel" roleURI="http://www.xbrl.org/2009/role/negatedLabel"  />
        <link:roleRef xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/role/negated-2009-12-16.xsd#negatedPeriodEndLabel" roleURI="http://www.xbrl.org/2009/role/negatedPeriodEndLabel"  />
        <link:roleRef xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/role/negated-2009-12-16.xsd#negatedPeriodStartLabel" roleURI="http://www.xbrl.org/2009/role/negatedPeriodStartLabel"  />
        <link:roleRef xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/role/negated-2009-12-16.xsd#negatedTotalLabel" roleURI="http://www.xbrl.org/2009/role/negatedTotalLabel"  />
        <link:roleRef xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/role/negated-2009-12-16.xsd#negatedNetLabel" roleURI="http://www.xbrl.org/2009/role/negatedNetLabel"  />
        <link:roleRef xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/role/negated-2009-12-16.xsd#negatedTerseLabel" roleURI="http://www.xbrl.org/2009/role/negatedTerseLabel"  />
        <link:roleRef xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/role/net-2009-12-16.xsd#netLabel" roleURI="http://www.xbrl.org/2009/role/netLabel"  />
        <link:presentationLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/cef/role/N2" >
          <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/cef/2025/cef-2025.xsd#cef_AllRisksMember" xlink:label="loc_cef_AllRisksMember_200074" />
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_RisksRelatedToThisOfferingMember" xlink:label="loc_tslx_RisksRelatedToThisOfferingMember_200581" />
          <link:presentationArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/parent-child" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_tslx_RisksRelatedToThisOfferingMember_200581" use="optional" order="1" preferredLabel="http://www.xbrl.org/2003/role/label" />
          <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_200072" />
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_RevolvingCreditsFacilityMember" xlink:label="loc_tslx_RevolvingCreditsFacilityMember_200587" />
          <link:presentationArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/parent-child" xlink:from="loc_us-gaap_ClassOfStockDomain_200072" xlink:to="loc_tslx_RevolvingCreditsFacilityMember_200587" use="optional" order="2" preferredLabel="http://www.xbrl.org/2003/role/label" />
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_TwoThousandTwentySixNotesMember" xlink:label="loc_tslx_TwoThousandTwentySixNotesMember_200588" />
          <link:presentationArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/parent-child" xlink:from="loc_us-gaap_ClassOfStockDomain_200072" xlink:to="loc_tslx_TwoThousandTwentySixNotesMember_200588" use="optional" order="3" preferredLabel="http://www.xbrl.org/2003/role/terseLabel" />
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_TwoThousandTwentyEightNotesMember" xlink:label="loc_tslx_TwoThousandTwentyEightNotesMember_200589" />
          <link:presentationArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/parent-child" xlink:from="loc_us-gaap_ClassOfStockDomain_200072" xlink:to="loc_tslx_TwoThousandTwentyEightNotesMember_200589" use="optional" order="4" preferredLabel="http://www.xbrl.org/2003/role/terseLabel" />
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_TwoThousandTwentyNineNotesMember" xlink:label="loc_tslx_TwoThousandTwentyNineNotesMember_200590" />
          <link:presentationArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/parent-child" xlink:from="loc_us-gaap_ClassOfStockDomain_200072" xlink:to="loc_tslx_TwoThousandTwentyNineNotesMember_200590" use="optional" order="5" preferredLabel="http://www.xbrl.org/2003/role/terseLabel" />
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_TwoThousandThirtyNotesMember" xlink:label="loc_tslx_TwoThousandThirtyNotesMember_200592" />
          <link:presentationArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/parent-child" xlink:from="loc_us-gaap_ClassOfStockDomain_200072" xlink:to="loc_tslx_TwoThousandThirtyNotesMember_200592" use="optional" order="6" preferredLabel="http://www.xbrl.org/2003/role/terseLabel" />
          <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_AddressTypeDomain" xlink:label="loc_dei_AddressTypeDomain_200069" />
        </link:presentationLink>
        <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:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/cef/role/SecurityOnly" >
          <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_200072" />
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_RevolvingCreditsFacilityMember" xlink:label="loc_tslx_RevolvingCreditsFacilityMember_200587" />
          <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_200072" xlink:to="loc_tslx_RevolvingCreditsFacilityMember_200587" use="optional" order="1" />
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_TwoThousandThirtyNotesMember" xlink:label="loc_tslx_TwoThousandThirtyNotesMember_200592" />
          <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_200072" xlink:to="loc_tslx_TwoThousandThirtyNotesMember_200592" use="optional" order="2" />
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_TwoThousandTwentyEightNotesMember" xlink:label="loc_tslx_TwoThousandTwentyEightNotesMember_200589" />
          <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_200072" xlink:to="loc_tslx_TwoThousandTwentyEightNotesMember_200589" use="optional" order="3" />
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_TwoThousandTwentyNineNotesMember" xlink:label="loc_tslx_TwoThousandTwentyNineNotesMember_200590" />
          <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_200072" xlink:to="loc_tslx_TwoThousandTwentyNineNotesMember_200590" use="optional" order="4" />
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_TwoThousandTwentySixNotesMember" xlink:label="loc_tslx_TwoThousandTwentySixNotesMember_200588" />
          <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_200072" xlink:to="loc_tslx_TwoThousandTwentySixNotesMember_200588" use="optional" order="5" />
        </link:definitionLink>
        <link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/cef/role/RiskOnly" >
          <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/cef/2025/cef-2025.xsd#cef_AllRisksMember" xlink:label="loc_cef_AllRisksMember_200074" />
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_RisksRelatedToThisOfferingMember" xlink:label="loc_tslx_RisksRelatedToThisOfferingMember_200581" />
          <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_tslx_RisksRelatedToThisOfferingMember_200581" use="optional" order="6" />
        </link:definitionLink>
        <link:labelLink xlink:type="extended" xlink:role="http://www.xbrl.org/2003/role/link" >
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_RisksRelatedToThisOfferingMember" xlink:label="loc_tslx_RisksRelatedToThisOfferingMember" />
          <link:label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="lab_tslx_RisksRelatedToThisOfferingMember" xml:lang="en-US">Risks Related to This Offering [Member]</link:label>
          <link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_tslx_RisksRelatedToThisOfferingMember" xlink:to="lab_tslx_RisksRelatedToThisOfferingMember" />
          <link:label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="lab_tslx_RisksRelatedToThisOfferingMember" xml:lang="en-US">Risks Related to This Offering.</link:label>
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_RevolvingCreditsFacilityMember" xlink:label="loc_tslx_RevolvingCreditsFacilityMember" />
          <link:label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="lab_tslx_RevolvingCreditsFacilityMember" xml:lang="en-US">Revolving Credits Facility [Member]</link:label>
          <link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_tslx_RevolvingCreditsFacilityMember" xlink:to="lab_tslx_RevolvingCreditsFacilityMember" />
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_TwoThousandThirtyNotesMember" xlink:label="loc_tslx_TwoThousandThirtyNotesMember" />
          <link:label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="lab_tslx_TwoThousandThirtyNotesMember" xml:lang="en-US">Two Thousand Thirty Notes [Member]</link:label>
          <link:label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="lab_tslx_TwoThousandThirtyNotesMember" xml:lang="en-US">2030 Notes</link:label>
          <link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_tslx_TwoThousandThirtyNotesMember" xlink:to="lab_tslx_TwoThousandThirtyNotesMember" />
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_TwoThousandTwentyEightNotesMember" xlink:label="loc_tslx_TwoThousandTwentyEightNotesMember" />
          <link:label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="lab_tslx_TwoThousandTwentyEightNotesMember" xml:lang="en-US">Two Thousand Twenty Eight Notes [Member]</link:label>
          <link:label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="lab_tslx_TwoThousandTwentyEightNotesMember" xml:lang="en-US">2028 Notes</link:label>
          <link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_tslx_TwoThousandTwentyEightNotesMember" xlink:to="lab_tslx_TwoThousandTwentyEightNotesMember" />
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_TwoThousandTwentyNineNotesMember" xlink:label="loc_tslx_TwoThousandTwentyNineNotesMember" />
          <link:label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="lab_tslx_TwoThousandTwentyNineNotesMember" xml:lang="en-US">Two Thousand Twenty Nine Notes [Member]</link:label>
          <link:label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="lab_tslx_TwoThousandTwentyNineNotesMember" xml:lang="en-US">2029 Notes</link:label>
          <link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_tslx_TwoThousandTwentyNineNotesMember" xlink:to="lab_tslx_TwoThousandTwentyNineNotesMember" />
          <link:loc xlink:type="locator" xlink:href="tslx-20260507.xsd#tslx_TwoThousandTwentySixNotesMember" xlink:label="loc_tslx_TwoThousandTwentySixNotesMember" />
          <link:label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="lab_tslx_TwoThousandTwentySixNotesMember" xml:lang="en-US">Two Thousand Twenty Six Notes [Member]</link:label>
          <link:label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="lab_tslx_TwoThousandTwentySixNotesMember" xml:lang="en-US">2026 Notes</link:label>
          <link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_tslx_TwoThousandTwentySixNotesMember" xlink:to="lab_tslx_TwoThousandTwentySixNotesMember" />
        </link:labelLink>
      </link:linkbase>
    </appinfo>
  </annotation>
  <import namespace="http://www.xbrl.org/2003/instance" schemaLocation="http://www.xbrl.org/2003/xbrl-instance-2003-12-31.xsd" />
  <import namespace="http://www.xbrl.org/2003/linkbase" schemaLocation="http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" />
  <import namespace="http://xbrl.org/2005/xbrldt" schemaLocation="http://www.xbrl.org/2005/xbrldt-2005.xsd" />
  <import namespace="http://xbrl.sec.gov/dei/2025" schemaLocation="https://xbrl.sec.gov/dei/2025/dei-2025.xsd" />
  <import namespace="http://www.xbrl.org/dtr/type/2024-01-31" schemaLocation="https://www.xbrl.org/dtr/type/2024-01-31/types.xsd" />
  <import namespace="http://xbrl.sec.gov/cef/2025" schemaLocation="https://xbrl.sec.gov/cef/2025/cef-2025.xsd" />
  <import namespace="http://xbrl.sec.gov/cef-pre/2025" schemaLocation="https://xbrl.sec.gov/cef/2025/cef-2025_pre.xsd" />
  <element name="RevolvingCreditsFacilityMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="tslx_RevolvingCreditsFacilityMember" substitutionGroup="xbrli:item" />
  <element name="TwoThousandThirtyNotesMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="tslx_TwoThousandThirtyNotesMember" substitutionGroup="xbrli:item" />
  <element name="TwoThousandTwentyEightNotesMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="tslx_TwoThousandTwentyEightNotesMember" substitutionGroup="xbrli:item" />
  <element name="TwoThousandTwentyNineNotesMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="tslx_TwoThousandTwentyNineNotesMember" substitutionGroup="xbrli:item" />
  <element name="TwoThousandTwentySixNotesMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="tslx_TwoThousandTwentySixNotesMember" substitutionGroup="xbrli:item" />
  <element name="RisksRelatedToThisOfferingMember" type="dtr-types:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" id="tslx_RisksRelatedToThisOfferingMember" substitutionGroup="xbrli:item" />
</schema>
