M80 was established in 1998 as an online company specialising in viral marketing. The agency's clients include Microsoft, SegaAmerica, Universal Music Group, Twentieth Century Fox and the NBA.
Rob Norman, chief executive of GroupM Interaction, said: "Our acquisition of M80 reflects our commitment to the value of peer-to-peer communication in marketing.
"Word-of-mouth at internet speed and user-generated content are of increasing importance, and M80 is already engaged with many of our clients and delivering huge value."
Norman has been charged with the growth of Group M in the digital and direct sectors. He was formerly chairman of Mediaedge:cia in the UK, before moving to the US to take up his new role.
The acquisition of online companies has recently become a high priority to media networks. Omnicom pooled-buying agency OPera last week said it was set to pool its £100m digital spend in negotiating with media owners through OPera Digital.
Aegis has also led the digital charge through its Isobar network, which plans and buys digital campaigns for clients including Renault, Danone and Adidas. Recent acquisitions include UK-based, full-service digital agency Glue and US search engine marketing company iProspect.
Group M combines the multi-territory buying resources of WPP agencies MediaCom, MindShare, Mediaedge:cia and Asia Pacific and Australasia-based agency Maxus.
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