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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2011-03-16</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000880643</issuerCik>
        <issuerName>GENTA INC DE/</issuerName>
        <issuerTradingSymbol>GNTAD</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001137918</rptOwnerCik>
            <rptOwnerName>ITRI LORETTTA M</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O GENTA INCORPORATED</rptOwnerStreet1>
            <rptOwnerStreet2>200 CONNELL DRIVE</rptOwnerStreet2>
            <rptOwnerCity>BERKELEY HEIGHTS</rptOwnerCity>
            <rptOwnerState>NJ</rptOwnerState>
            <rptOwnerZipCode>07922</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Pres RD Chief Medical Officer</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1180204</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>229372275</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Spouse</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Includes 52,622 shares of common stock potentially issuable upon the vesting of currently outstanding restricted stock units held by Dr. Itri, 1,025,070 shares of common stock issuable upon the conversion of Senior Convertible Promissory Notes due September 4, 2011, as amended (the Notes) held by Dr. Itri's IRA and 102,512 shares of common stock issuable upon the exercise of December 2010 Warrants held by Dr. Itri's IRA.</footnote>
        <footnote id="F2">Includes 199,580 shares of common stock held in joint account with Dr. Itri's spouse, Dr. Warrell, 119,420 shares of common stock potentially issuable upon the vesting of currently outstanding restricted stock units held by Dr. Warrell, 118,579,395 shares of common stock issuable upon the conversion of Notes held in joint account with Dr. Warrell, 683,380 shares of common stock issuable upon the conversion of Notes held by Dr. Warrell's IRA, 97,045,892 shares of common stock issuable upon the exercise of March 2010 Warrants held in joint account with Dr. Warrell, 12,676,270 shares of common stock issuable upon the exercise of December 2010 Warrants held in joint account with Dr. Warrell and 68,338 shares of common stock issuable upon the exercise of December 2010 Warrants held by Dr. Warrell's IRA.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Loretta M. Itri</signatureName>
        <signatureDate>2011-03-16</signatureDate>
    </ownerSignature>
</ownershipDocument>
