Publicis has made some smaller acquisitions in marketing services, an area that includes public relations and digital services and which is growing faster than traditional advertising, but some analysts had been hoping for a deal with more impact.
"This operation will constitute a powerful growth engine for all of Publicis Groupe in a context in which digital and interactive marketing and communications should represent more than 10% of all worldwide investment by 2010," Publicis Chairman and Chief Executive Maurice Levy said in a statement.
Publicis shares had closed up 2.7% to 30.73 euros in Paris before the announcement.
The deal has been approved unanimously by both companies' boards.
Digitas clients include American Express, General Motors, Home Depot and Time Warner. The company is based in Boston.
Publicis said it expected the deal to boost its earnings on an operating margin per share basis by about 4% in 2007 and by about 6 percent in 2008. Publicis defines operating profit as earnings before interest, taxes and amortisation.
Publicis added that its 16.7% operating margin target for 2008 remains unchanged.
Messier & Associates and Citigroup advised Publicis while Bear Stearns advised Digitas.