"I was happy at Manchester for seven or eight years, it was a big club for me... but now it's a new challenge and the time to change," the 25-year-old told Fiorentina's Viola Channel website on Saturday.
"Fiorentina only missed out on Champions League qualification by two points last season, so this year I want to help them get into the Champions League."
The winger joins former Juventus and AC Milan striker Alessandro Matri among Fiorentina's new faces as the club look to reinforce their attack with forwards Giuseppe Rossi and Mario Gomez out with knee injuries.
Fiorentina are fourth in Serie A. They are five points behind Napoli, who are in the third Champions League spot, and 15 points behind leaders Juventus.