Publicis, the world's third-largest advertising agency, has agreed to buy digital ad specialist Sapient for $3.7 billion in cash as it seeks to accelerate growth after a failed merger earlier this year. 

Publicis said the deal values US-based Sapient at $25 a share, which represented a 44% premium to Friday's close. 

The deal will be financed through its existing cash and new debt. 

For Publicis chief executive Maurice Levy, the deal is part of a push to revitalise the group at a time when its quarterly top-line growth has lagged rivals WPP and Interpublic among others. 

Levy has blamed the poor performance on the hangover from Publicis' failed "merger of equals" with the world's second biggest ad agency Omnicom, which was announced in August 2013 and abandoned in May over control and cultural clashes. 

Levy is betting that Sapient - which earned 63% of 2013 sales in the healthy ad market of North America and has 13,000 employees - will help Publicis get back on its feet. 

"Publicis and Sapient will be a technology leader to help our clients go digital," Levy said. The deal will create a foundation for accelerated growth," he said, by giving it access to new markets and revenue stream. 

The deal will speed Publicis' roughly seven year old effort to earn more revenue from so-called digital advertising, which includes everything from online marketing to brand building on social networks and automatic ad buying for major customers. 

Last year, 38.4% of Publicis' sales came from digital, and it had been aiming to reach 50% by 2018, something that the Sapient deal will make happen immediately. 

Sapient counts global corporations, including car maker Fiat, consumer products group Unilever, and retailer Marks & Spencer among its customers. 

Publicis did not say when the Sapient acquisition would add to group profits but expects €50m in annual cost savings from the combination. 

Its management and supervisory boards unanimously backed the deal, as did the board of Sapient, which will recommend shareholders tender their shares. As a result, Sapient will be de-listed from the Nasdaq stock exchange. 

Sapient boss Alan Herrick will continue to run the company and is to join Publicis' management team, while Jerry Greenberg, the co-chairman of Sapient's board will become a board member of Publicis. 

The transaction is expected to close in the first quarter of next year.