The deal, reported to be worth an estimated $600m (£476m) by PRWeek sister title Campaign, includes media relations, comms, public affairs and traditional and digital advertising services. The agency will create a 'Team WBA' to service the business.
WPP's PR agencies, which include Cohn&Wolfe, Burson-Marsteller, Ogilvy PR, Hill & Knowlton Strategies and Finsbury, plus several others, could be in line to pick up substantial briefs.
A company spokesperson declined to confirm whether WPP-owned PR agencies would have to navigate an internal pitch process to win relevant aspects of the Walgreens business.
However, WPP chief executive Sir Martin Sorrell said: "Team WBA will be a custom solution that draws together the talent and resources from across WPP to develop effective communications that will differentiate WBA and drive results."
As part of the deal, WPP will also create 'hub' offices in Chicago, London and New York to support Walgreens in the US, Boots in the UK and its Alliance Healthcare businesses, the agency said.
Walgreens and Alliance Boots merged in December 2014, following a deal worth a reported £16bn. The company’s CEO Stefano Pessina has a 16% stake in the business.
On the WPP deal, he said: "We have grown rapidly, so too have our marketing and communications needs, and this multi-dimensional model and new way of working will better support our vision for the company and future growth plans."