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

    <schemaVersion>X0607</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2026-05-13</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001362468</issuerCik>
        <issuerName>Allegiant Travel CO</issuerName>
        <issuerTradingSymbol>ALGT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001593661</rptOwnerCik>
            <rptOwnerName>Kennedy Thomas C</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerNonUSAddressFlag>false</rptOwnerNonUSAddressFlag>
            <rptOwnerStreet1>1201 N TOWN CENTER DR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>LAS VEGAS</rptOwnerCity>
            <rptOwnerState>NV</rptOwnerState>
            <rptOwnerZipCode>89144</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5894</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">On May 13, 2026, pursuant to the Agreement and Plan of Merger dated as of January 11, 2026 (the &quot;Merger Agreement&quot;), by and among the Issuer, Sun Country Airlines Holdings, Inc., (&quot;Sun Country&quot;), Mirage Merger Sub, Inc. (&quot;Merger Sub 1&quot;), a direct wholly owned subsidiary of the Issuer, and Sawdust Merger Sub, LLC (&quot;Merger Sub 2&quot;), a direct wholly owned subsidiary of the Issuer, (i) Merger Sub 1 merged with and into Sun Country (the &quot;First Merger&quot;), with Sun Country surviving the First Merger as a direct wholly owned subsidiary of the Issuer and (ii) immediately after the First Merger, Sun Country merged with and into Merger Sub 2, with Merger Sub 2 surviving as a direct, wholly owned subsidiary of the Issuer (the &quot;Second Merger&quot; and, together with the First Merger, the &quot;Mergers&quot;).</footnote>
        <footnote id="F2">Pursuant to the Merger Agreement, as of immediately prior to the effective time of the First Merger (the &quot;First Effective Time&quot;), each share of Sun Country's common stock, par value $0.01 per share, was converted into the right to receive (i) $4.10 in cash, without interest and (ii) 0.1557 shares of the Issuer's common stock, par value $0.001 per share (collectively, the &quot;Merger Consideration&quot;). As of immediately prior to the First Effective Time, each Sun Country equity award held by the reporting person became fully vested (to the extent not yet vested), cancelled and converted into the right to receive the Merger Consideration.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Robert B. Goldberg, under power of attorney</signatureName>
        <signatureDate>2026-05-15</signatureDate>
    </ownerSignature>
</ownershipDocument>
