Publicis has paid $286.8m in cash plus 6.5m ordinary shares for Razorfish, giving Microsoft a 3.3% stake in Publicis.
The deal means Publicis has effectively already reached its stated target of achieving 25% of its revenue from digital activities a year ahead of schedule.
Razorfish, which counts Levi's and McDonald's among its clients, will be a wholly-owned subsidiary of Publicis Groupe, and will fall under its global network VivaKi, the "media and digital umbrella" encompassing Starcom MediaVest Group, ZenithOptimedia, Denuo, Digitas and VivaKi Nerve Center.
VivaKi managing partner David Kenny, a member of the Publicis Groupe Management Board, has been charged with integrating Razorfish into the organisation.
In a company statement this morning, no mention was made of a reported ongoing agreement with Microsoft, under which Publicis would spend a proportion of its clients' money with Microsoft in return for improved rates.
It has previously been suggested the commitment could be as high as $3bn over the next five years, though this has been denied by Publicis.
There was also no mention of how it would deal with potential conflicts of interest with its other digital network, Digitas, which it acquired for $1.6bn in 2007.
It is thought, however, that some services will be shared on an occasional basis, with the two acting as competitors in markets where client conflict may be an issue.