The 29-year-old striker has joined the Viola until the end of the season.
A statement from Fiorentina read: "Fiorentina announces it has acquired on a temporary basis the sporting rights of Alessandro Matri from AC Milan."
The 29-year-old, who only moved to Milan last summer from Juventus, has scored just once in 15 Serie A appearances this campaign for the Rossoneri.
Fiorentina, who are competing in Serie A, Coppa Italia and the Europa League, were desperate to strengthen their attacking line after the team's top scorer Giuseppe Rossi joined German forward Mauro Gomez on the sidelines earlier this month.
Rossi sustained a knee injury and is expected to be out of action for three months.
Matri is expected to make his Fiorentina debut in Sunday's game at rock-bottom Catania.