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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2019-02-25</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001239784</rptOwnerCik>
            <rptOwnerName>ETKIND ANDREW R</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>MUEHLENTALSTRASSE 2</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SCHAFFHAUSEN</rptOwnerCity>
            <rptOwnerState>V8</rptOwnerState>
            <rptOwnerZipCode>8200</rptOwnerZipCode>
            <rptOwnerStateDescription>SWITZERLAND</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isOfficer>1</isOfficer>
            <officerTitle>VP, General Counsel, Secretary</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Registered Shares</value>
            </securityTitle>
            <transactionDate>
                <value>2019-02-25</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>5772</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>56741</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Registered Shares</value>
            </securityTitle>
            <transactionDate>
                <value>2019-02-25</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1909</value>
                    <footnoteId id="F3"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>83.55</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>54832</value>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Shares acquired pursuant to a restricted stock units award granted in February 2018 that had both performance-based and time-based vesting conditions. The performance-based vesting conditions have been satisfied, and now the award is subject only to time-based vesting conditions. The award vests in three equal annual installments beginning on February 25, 2019.</footnote>
        <footnote id="F2">Includes (i) 13,166 unvested shares acquired pursuant to previously granted restricted stock unit awards, (ii) the 5,772 unvested shares acquired pursuant to the restricted stock units award described in Footnote 1 and (iii) 60 shares acquired in December 2018 under the Garmin Ltd. Employee Stock Purchase Plan.</footnote>
        <footnote id="F3">1,924 shares that were acquired by the reporting person pursuant to the restricted stock units award described in Footnote 1 and 3,354 shares that were acquired by the reporting person pursuant to other previously granted restricted stock unit awards vested and were paid to the reporting person. Of these shares, 1,909 shares were withheld to pay a resulting tax liability.</footnote>
        <footnote id="F4">Includes 9,812 unvested shares acquired pursuant to previously granted restricted stock unit awards and 3,848 remaining unvested shares acquired pursuant to the restricted stock units award described in Footnote 1.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Andrew R. Etkind</signatureName>
        <signatureDate>2019-02-27</signatureDate>
    </ownerSignature>
</ownershipDocument>
