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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2022-12-15</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001801889</rptOwnerCik>
            <rptOwnerName>Munn Matthew</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>28125 CABOT DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 120</rptOwnerStreet2>
            <rptOwnerCity>NOVI</rptOwnerCity>
            <rptOwnerState>MI</rptOwnerState>
            <rptOwnerZipCode>48377</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isOfficer>1</isOfficer>
            <officerTitle>VP, Man. Director - Auto OEM</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Registered Shares</value>
            </securityTitle>
            <transactionDate>
                <value>2022-12-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1797</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>92.96</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>11340</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Registered Shares</value>
            </securityTitle>
            <transactionDate>
                <value>2022-12-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>580</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>92.72</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>10760</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Registered Shares</value>
            </securityTitle>
            <transactionDate>
                <value>2022-12-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>3723</value>
                    <footnoteId id="F4"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>14483</value>
                    <footnoteId id="F5"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Includes 8,244 unvested shares acquired pursuant to previously granted restricted stock unit awards.</footnote>
        <footnote id="F2">On December 15, 2022, 2,020 shares that were acquired pursuant to previously granted restricted stock unit awards vested and were released to the reporting person. Of those 2,020 shares, 580 shares were withheld to pay the resulting tax liability.</footnote>
        <footnote id="F3">Includes 6,224 unvested shares acquired pursuant to previously granted restricted stock unit awards.</footnote>
        <footnote id="F4">On December 15, 2022 the reporting person received a grant of 3,723 restricted stock units, which vest in three equal annual installments, beginning on December 15, 2023.</footnote>
        <footnote id="F5">Includes 9,947 unvested shares acquired pursuant to the December 15, 2022 restricted stock unit award and previously granted restricted stock unit awards.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Paul E. Cassat, Attorney-in-Fact</signatureName>
        <signatureDate>2022-12-19</signatureDate>
    </ownerSignature>
</ownershipDocument>
