The 29-year-old moves to Florence after struggling to find the net during his second spell at San Siro, scoring just one goal in 18 appearances for Milan this season.
And the former Juventus forward, who left Turin to re-join Milan in August, will look to revitalise his career at the Stadio Artemio Franchi, as Fiorentina aim to keep pace in the race for UEFA Champions League football.
Matri's move bolsters a forward line that suffered a blow earlier this month when Serie A top scorer Giuseppe Rossi suffered ligament damage in the 1-0 win over Livorno on January 5.
Fiorentina sit fourth in Serie A after 19 games, five points adrift of third-placed Napoli.
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