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

    <schemaVersion>X0304</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2011-11-03</periodOfReport>

    <dateOfOriginalSubmission>2011-11-03</dateOfOriginalSubmission>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001084580</issuerCik>
        <issuerName>JEFFERIES GROUP INC /DE/</issuerName>
        <issuerTradingSymbol>JEF</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0000924255</rptOwnerCik>
            <rptOwnerName>STEINBERG JOSEPH S</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O LEUCADIA NATIONAL CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>315 PARK AVENUE SOUTH</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10010</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2011-11-03</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1000000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>11.8439</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>57506024</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Explanation</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>18729</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Reflects weighted average per share purchase price of separately priced transactions at a range of $11.48 - $12.06.  Upon the request of the Commission or a security holder of the issuer, full information regarding the number of shares purchased at each separate price will be provided to the requesting party.</footnote>
        <footnote id="F2">Reflects shares of Issuer's common stock beneficially owned by Leucadia National Corporation (&quot;Leucadia&quot;). The Reporting Person is a significant shareholder, director and President of Leucadia.  The Reporting Person disclaims beneficial ownership of the Issuer's common stock beneficially owned by Leucadia, except to the extent of his pecuniary interest, if any.</footnote>
    </footnotes>

    <remarks>This amendment is being filed to include Footnote 1.</remarks>

    <ownerSignature>
        <signatureName>/s/ Joseph A. Orlando, attorney in fact</signatureName>
        <signatureDate>2011-11-07</signatureDate>
    </ownerSignature>
</ownershipDocument>
