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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2021-02-25</periodOfReport>

    <issuer>
        <issuerCik>0001121788</issuerCik>
        <issuerName>GARMIN LTD</issuerName>
        <issuerTradingSymbol>GRMN</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001483374</rptOwnerCik>
            <rptOwnerName>Biddlecombe Sean</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>LIBERTY HOUSE</rptOwnerStreet1>
            <rptOwnerStreet2>HOUNSDOWN BUSINESS PARK</rptOwnerStreet2>
            <rptOwnerCity>SOUTHAMPTON</rptOwnerCity>
            <rptOwnerState>X0</rptOwnerState>
            <rptOwnerZipCode>SO409LR</rptOwnerZipCode>
            <rptOwnerStateDescription>UNITED KINGDOM</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isOfficer>1</isOfficer>
            <officerTitle>Managing Director, EMEA</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Registered Shares</value>
            </securityTitle>
            <transactionDate>
                <value>2021-02-25</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>746</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>124.34</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>9389</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">1,582 shares that were acquired by the reporting person pursuant to previously granted restricted stock unit awards vested and were paid to the reporting person. Of these shares, 746 shares were withheld to pay a resulting tax liability.</footnote>
        <footnote id="F2">Includes 3,564 unvested shares acquired pursuant to previously granted restricted stock unit awards.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Andrew R. Etkind, Attorney-in-Fact</signatureName>
        <signatureDate>2021-03-01</signatureDate>
    </ownerSignature>
</ownershipDocument>
