<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2022-10-26</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001913721</issuerCik>
        <issuerName>AGTB Private BDC</issuerName>
        <issuerTradingSymbol>N/A</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0000860662</rptOwnerCik>
            <rptOwnerName>ANGELO GORDON &amp; CO., L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>245 PARK AVENUE, 26TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10167</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001944208</rptOwnerCik>
            <rptOwnerName>AGTB BDC Holdings, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ANGELO, GORDON &amp; CO., L.P.</rptOwnerStreet1>
            <rptOwnerStreet2>245 PARK AVENUE, 26TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10167</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001943685</rptOwnerCik>
            <rptOwnerName>AGTB BDC Holdings GP LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ANGELO, GORDON &amp; CO., L.P.</rptOwnerStreet1>
            <rptOwnerStreet2>245 PARK AVENUE, 26TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10167</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle></officerTitle>
            <otherText>Reporting Person</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001064683</rptOwnerCik>
            <rptOwnerName>AG GP LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ANGELO, GORDON &amp; CO., L.P.</rptOwnerStreet1>
            <rptOwnerStreet2>245 PARK AVENUE, 26TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10167</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle></officerTitle>
            <otherText>Reporting Person</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001320216</rptOwnerCik>
            <rptOwnerName>Baumgarten Joshua</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ANGELO, GORDON &amp; CO., L.P.</rptOwnerStreet1>
            <rptOwnerStreet2>245 PARK AVENUE, 26TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10167</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle></officerTitle>
            <otherText>Reporting Person</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001872317</rptOwnerCik>
            <rptOwnerName>SCHWARTZ ADAM R</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ANGELO, GORDON &amp; CO., L.P.</rptOwnerStreet1>
            <rptOwnerStreet2>245 PARK AVENUE, 26TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10167</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle></officerTitle>
            <otherText>Reporting Person</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common shares of beneficial interest</value>
            </securityTitle>
            <transactionDate>
                <value>2022-10-26</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>32841.788</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>25.548</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>158610.258</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F1"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common shares of beneficial interest</value>
            </securityTitle>
            <transactionDate>
                <value>2022-10-26</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2031544.544</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>25.548</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>8902078.502</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">These securities are held directly by Angelo Gordon. Josh Baumgarten and Adam Schwartz are the co-managing members of AG GP, which is the sole general partner of Angelo Gordon. Each of Messrs. Baumgarten and Schwartz, and AG GP may be deemed to control Angelo Gordon. Each of AG GP and Messrs. Baumgarten and Schwartz disclaim beneficial ownership of the securities reported herein except to the extent of its or his pecuniary interest therein and this report shall not be deemed to be an admission that any of AG GP, Mr. Baumgarten or Mr. Schwartz is the beneficial owner of such securities for purposes of Section 16 or for any other purpose.</footnote>
        <footnote id="F2">These securities are held directly by AGTB BDC Holdings, L.P. (&quot;BDC Holdings&quot;). AGTB BDC Holdings GP LLC (&quot;BDC Holdings GP&quot;) is the sole general partner of BDC Holdings, Angelo, Gordon &amp; Co., L.P. (&quot;Angelo Gordon&quot;) is the investment advisor to BDC Holdings and sole member of BDC Holdings GP, AG GP LLC (&quot;AG GP&quot;) is the sole general partner of Angelo Gordon, and Mr. Baumgarten and Mr. Schwartz are the co-managing members of AG GP. Each of Messrs. Baumgarten and Schwartz, and AG GP may be deemed to control Angelo Gordon. Each of BDC Holdings GP, Angelo Gordon, AG GP and Messrs. Baumgarten and Schwartz disclaim beneficial ownership of the securities reported herein except to the extent of its or his pecuniary interest therein and this report shall not be deemed to be an admission that any of BDC Holdings GP, Angelo Gordon, AG GP, Mr. Baumgarten or Mr. Schwartz is the beneficial owner of such securities for purposes of Section 16 or for any other purpose.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Christopher D. Moore, as Attorney-in-Fact for Josh Baumgarten</signatureName>
        <signatureDate>2022-10-28</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Christopher D. Moore, as Attorney-in-Fact for Josh Baumgarten and Adam Schwartz, Managing Members of AG GP LLC, Managing Members of Angelo, Gordon &amp; Co., L.P., Investment Advisor of AGTB BDC Holdings, L.P.</signatureName>
        <signatureDate>2022-10-28</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Christopher D. Moore, as Attorney-in-Fact for Josh Baumgarten and Adam Schwartz, Managing Members of AG GP LLC, Managing Members of Angelo, Gordon &amp; Co., L.P.</signatureName>
        <signatureDate>2022-10-28</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Christopher D. Moore, as Attorney-in-Fact for Adam Schwartz</signatureName>
        <signatureDate>2022-10-28</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Christopher D. Moore, as Attorney-in-Fact for Josh Baumgarten and Adam Schwartz, Managing Members of AG GP LLC, Managing Members of Angelo, Gordon &amp; Co., L.P., Sole Member of AGTB BDC Holdings GP LLC</signatureName>
        <signatureDate>2022-10-28</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Christopher D. Moore, as Attorney-in-Fact for Josh Baumgarten and Adam Schwartz, Managing Members of AG GP LLC</signatureName>
        <signatureDate>2022-10-28</signatureDate>
    </ownerSignature>
</ownershipDocument>
