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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2014-03-07</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0001473597</issuerCik>
        <issuerName>STR HOLDINGS, INC.</issuerName>
        <issuerTradingSymbol>STRI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0000824468</rptOwnerCik>
            <rptOwnerName>CREDIT SUISSE AG/</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>SEE FOOTNOTE (1) IN EXHIBIT 99.1</rptOwnerStreet1>
            <rptOwnerStreet2>11 MADISON AVENUE</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10010</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $0.01 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2014-03-07</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>10079708</value>
                    <footnoteId id="F3"/>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>1.54</value>
                    <footnoteId id="F3"/>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F1"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">See Footnote 1 on Exhibit 99.1</footnote>
        <footnote id="F2">See Footnote 2 on Exhibit 99.1</footnote>
        <footnote id="F3">The amount reported represents the number of shares tendered by the Reporting Person in the Issuer's modified &quot;Dutch auction&quot; tender offer.</footnote>
        <footnote id="F4">Excludes 745,572 shares held by Credit Suisse/CFIG STR Investors SPV, LLC previously but no longer reported by Credit Suisse as a result of the sale of the CFIG business in January 2014.</footnote>
    </footnotes>

    <remarks>See Exhibit 99.1.</remarks>

    <ownerSignature>
        <signatureName>/s/ Ivy Dodes, Managing Director, on behalf of the Reporting Person</signatureName>
        <signatureDate>2014-03-11</signatureDate>
    </ownerSignature>
</ownershipDocument>
