It had been rumoured that the 35-year-old forward would be joining Tuscan rivals Siena, but Fiorentina officials made a late swoop for their former player, who has signed a one-year deal.
"ACF Fiorentina have acquired the rights of Luca Toni. He returns to Fiorentina after five years," the club said.
Vincenzo Montella's side were in the market for a striker after missing out on Bulgaria international Dimitar Berbatov, who looked set to join the club on Wednesday before backing out at the last minute.
Toni became the first person in almost half a century to score more than 30 Serie A goals when he netted 31 during the 2005-2006 season.
During his two years at the Tuscan club, he scored 47 times in 67 league games.
Toni returns to Serie A as a free agent after leaving Dubai club Al Nasr at the end of the last UAE Football League season.
Fiorentina play Al Nasr in a friendly on September 7 in Florence.
- Sports & Recreation
- Luca Toni