The Mali international will spend the rest of the season on loan in Florence, and the Viola could make his move from Paris Saint-Germain permanent should they see fit
La Viola announced this week that they were in talks with the Ligue 1 leaders to bring the former Juventus player back to Serie A during the winter transfer window.
Talks with the 28-year-old have come to fruition, and Fiorentina could elect to sign the player permanently once his loan period expires.
"Fiorentina announce that they have acquired the rights to the player Mohamed Sissoko from Paris Saint-Germain," a statement on the club's official website read.
"The player comes on loan with the right to purchase."
The 28-year-old began his career with Auxerre, and enjoyed spells with Valencia, Liverpool and Juve before moving to the Parc des Princes.