The 28-year-old, who spent three years in Italy with Juventus, is expected to complete a move to the Serie A outfit before the end of the month, according to Fiorentina sporting director Daniele Prade.
"We decided to sign a player with different characteristics to the ones we have in our present squad. We thought about signing him a while ago. Momo is a highly motivated player and we are happy to bring him in," Prade told the club's official website.
It is understood that Sissoko, who has also played for Liverpool, will join initially on loan before completing a permanent move at the end of the season.