The Viola have turned to the former Juventus forward to strengthen their attack following the loss of star striker Giuseppe Rossi to a knee injury.
The Italy international, who began his professional career at San Siro, rejoined the Rossoneri from Juventus for a reported 12 million euros last summer.
However, Matri managed just one goal in 15 Serie A appearances for Milan and will now spend the rest of the 2013-14 campaign at the Artemio Franchi.
"Fiorentina announce that they have acquired, on a temporary basis, the rights to Alessandro Matri of AC Milan," read a statement on the Viola's official website.
The Florence-based side was in need of attacking cover in light of the recent knee injury suffered by star striker Giuseppe Rossi.
It has been reported in Italy that Matri, who will be formally presented to the press on Thursday afternoon, could soon be joined at Fiorentina by Manchester United midfielder Anderson.