The former Parma hit-man has been playing second fiddle at the Rossoneri for the past couple of seasons and he has spent the majority of the campaign on the bench.
His performances were woeful and he has been quickly shipped out to La Viola who have confirmed the deal.
A statement on the club's official website reads: "Fiorentina would like to communicate that the club has signed Alberto Gilardino from AC Milan on a permanent basis.
"Gilardino will be unveiled to the press at 12.30 on Thursday at the Artemio Franchi stadium."
Reports in Italy claim the Viola paid €15m for the striker, he will earn €1.65m per season plus bonuses and he has signed a five year deal.
The striker is Fiorentina's third signing of the current transfer window following the arrivals of Felipe Melo and Stevan Jovetic.