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

Publicis said on Monday that the deal values U.S.-based Sapient at $25.00 per share, which represented a 44 percent premium to Friday's close.

For Publicis Chief Executive Maurice Levy, the deal is part of a push to revitalize 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 world number 2 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 percent of 2013 sales in the healthy ad market of North America, will help Publicis get back on its feet.

"It will give Publicis access to new markets and create new revenue streams," said the veteran CEO in a statement.

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

Publicis' management and supervisory boards unanimously backed the deal, as did the board of Sapient, which will recommend shareholders tender their shares.

The transaction is expected to close in the first quarter of next year. Citigroup has committed to financing the bid.

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