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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2015-11-01</periodOfReport>

    <issuer>
        <issuerCik>0001618921</issuerCik>
        <issuerName>Walgreens Boots Alliance, Inc.</issuerName>
        <issuerTradingSymbol>WBA</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001020079</rptOwnerCik>
            <rptOwnerName>FOOTE WILLIAM C</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O WALGREENS BOOTS ALLIANCE, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>108 WILMOT ROAD</rptOwnerStreet2>
            <rptOwnerCity>DEERFIELD</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60015</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Phantom Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2015-11-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2243.741</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>84.68</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F2"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>2243.741</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>43959.2</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The phantom stock is issued as non-employee director compensation under the Walgreens Boots Alliance, Inc. 2013 Omnibus Incentive Plan, as amended (including the applicable election forms thereunder, the &quot;Plan&quot;), and each unit of phantom stock is the economic equivalent of one share of the company's common stock.</footnote>
        <footnote id="F2">To be settled following termination of service as a director in accordance with the terms and conditions of the Plan as in effect from time to time.</footnote>
        <footnote id="F3">Includes phantom stock units issued in lieu of dividends (through September 30, 2015) on outstanding phantom stock units.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Kelsey Chin, attorney-in-fact</signatureName>
        <signatureDate>2015-11-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2015-11-01
0001618921
Walgreens Boots Alliance, Inc.
WBA
0001020079
FOOTE WILLIAM C
C/O WALGREENS BOOTS ALLIANCE, INC.
108 WILMOT ROAD
DEERFIELD
IL
60015
1
0
0
0
Phantom Stock Units
2015-11-01
4
A
0
2243.741
84.68
A
Common Stock
2243.741
43959.2
D
The phantom stock is issued as non-employee director compensation under the Walgreens Boots Alliance, Inc. 2013 Omnibus Incentive Plan, as amended (including the applicable election forms thereunder, the "Plan"), and each unit of phantom stock is the economic equivalent of one share of the company's common stock.
To be settled following termination of service as a director in accordance with the terms and conditions of the Plan as in effect from time to time.
Includes phantom stock units issued in lieu of dividends (through September 30, 2015) on outstanding phantom stock units.
/s/ Kelsey Chin, attorney-in-fact
2015-11-03