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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2011-01-13</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000880643</issuerCik>
        <issuerName>GENTA INC DE/</issuerName>
        <issuerTradingSymbol>GNTA</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 R&amp;D Chief Medical Officer</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>7634855</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1113283564</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 2,631,203 shares of common stock potentially issuable upon the vesting of currently outstanding restricted stock units held by Dr. Itri, 4,548,750 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, and 454,902 shares of stock issuable upon the exercise of the December 2010 Warrant.</footnote>
        <footnote id="F2">Includes 7,863,781 shares of common stock held in joint account with Dr. Itri's spouse, Dr. Warrell, 5,981,681 shares of common stock potentially issuable upon the vesting of currently outstanding restricted stock units held by Dr. Warrell, 542,906,641 shares of common stock issuable upon the conversion of Notes held in joint account with Dr. Warrell, 3,032,500 shares of common stock issuable upon the conversion of Notes held by Dr. Warrell's IRA, 496,944,714 shares of common stock issuable upon the exercise of the March 2010 Warrant held in joint account with Dr. Warrell, 56,250,980 shares of common stock issuable upon the exercise of the December 2010 Warrant held in joint account with Dr. Warrell, and 303,267 shares of common stock issuable upon the exercise of December 2010 Warrant held by Dr. Warrell's IRA.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Loretta M. Itri</signatureName>
        <signatureDate>2011-01-13</signatureDate>
    </ownerSignature>
</ownershipDocument>
