The pharmacy giant, which runs the Walgreens chains in the US and Boots' branches in the UK, spends about $600m (£500m) a year on advertising.
It is thought a deal has been agreed, but it was not clear which WPP agencies might work on Walgreens Boots Alliance or if the company might retain some of its existing agencies as Campaign went to press.
A spokesman for Walgreens Boots Alliance declined to offer any comment or guidance.
Industry sources have been speculating for months that Walgreens Boots Alliance’s business was "in play".
Boots has used Omnicom's OMD UK for media in the UK since 2008. Walgreens separately appointed OMD in the US in 2013 – a year before the chain merged with Boots.
Independent shop Mother has been the UK creative agency for Boots since 2003, while Omnicom's GSD&M is the US creative agency of record.
Walgreens Boots Alliance spent $598m on global advertising in its most recent financial year, according to a stock market filing, up from $491m in 2015 and $265m in 2014 when the group was smaller.
A win for WPP would be a tonic for the world’s biggest ad group that suffered a string of major media account losses, including Volkswagen globally and AT&T in the US, last year.
WPP referred enquiries to Walgreens Boots Alliance.