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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2015-03-17</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001343719</issuerCik>
        <issuerName>Energy XXI Ltd</issuerName>
        <issuerTradingSymbol>EXXI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001481755</rptOwnerCik>
            <rptOwnerName>LACHANCE JAMES</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1021 MAIN STREET</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 2626</rptOwnerStreet2>
            <rptOwnerCity>HOUSTON</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77002</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Interim Chief Strategic Officr</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2015-03-12</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1644737</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1644737</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1644737</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On March 17, 2015, the restricted stock units were deemed awarded as of March 12, 2015, in settlement of the $5,000,000 success fee payable under the Interim Chief Strategic Officer Agreement effective as of January 15, 2015 ( the &quot;Agreement&quot;), at the conversion price of $3.04 as set forth in the Agreement. All restricted stock units granted to Mr. LaChance will be settled in a lump sum cash payment in an amount equal to the number of restricted stock units multiplied by the per share closing price of the Issuer's common stock on the Settlement Date (the &quot;RSU Settlement Payment&quot;); provided, however, Mr. LaChance may elect to purchase from the Issuer, in lieu of receipt of all or a portion of the RSU Settlement Payment in cash, a number of shares of the Issuer's common stock equal to (x) all or such portion, as applicable, of the RSU Settlement Payment divided by (y) the closing price of the Issuer's common stock on the Settlement Date.</footnote>
        <footnote id="F2">The restricted stock units are payable on the earliest to occur of: (x) March 12, 2016, (y) a &quot;change in the ownership or effective control&quot; or &quot;a change in the ownership of a substantial portion of the assets&quot; of EXXI (as defined in Treasury Regulation ? 1.409A-3(i)(5)) and (z) the date that Mr. LaChance incurs a &quot;separation from service&quot; with the Issuer (as defined in Treasury Regulation ? 1.409A-1(h)) (the &quot;Settlement Date&quot;).</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/S/ JAMES LACHANCE</signatureName>
        <signatureDate>2015-03-17</signatureDate>
    </ownerSignature>
</ownershipDocument>
