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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2013-03-01</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000096223</issuerCik>
        <issuerName>LEUCADIA NATIONAL CORP</issuerName>
        <issuerTradingSymbol>LUK</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>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chairman of the Board</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Shares</value>
            </securityTitle>
            <transactionDate>
                <value>2013-03-01</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>20523</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2292688</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Shares</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>21241306</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Explanation</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Reflects acquisition pursuant to a merger agreement between Leucadia National Corporation (&quot;Leucadia&quot;) and Jefferies Group, Inc. (&quot;Jefferies&quot;) in which each share of Jefferies common stock was exchanged for 0.81 Leucadia common shares.  The closing price of a Leucadia common share on March 1, 2013 (the effective date of the merger) was $26.40 and the closing price of a share of Jefferies common stock on February 28, 2013 (the last trading day prior to effectiveness of the merger) was $21.72.</footnote>
        <footnote id="F2">Includes 139,200 Leucadia common shares beneficially owned by the Reporting Person's wife and daughter, 18,762,394 Leucadia common shares held by corporations that are wholly owned by the Reporting Person, or held by corporations that are wholly owned by family trusts as to which the Reporting Person has sole voting and dispositive control, or held by such trusts, 2,339,712 Leucadia common shares held in a trust for the benefit of the Reporting Person's children as to which the Reporting Person may be deemed to be the beneficial owner and 1,200,000 Leucadia common shares which the Reporting Person has the right to acquire upon exercise of warrants.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Joseph S. Steinberg</signatureName>
        <signatureDate>2013-03-05</signatureDate>
    </ownerSignature>
</ownershipDocument>
