The 35-year-old returns to the Stadio Artemio Franchi after a five-year absence. Toni enjoyed a prolific spell in Tuscany before signing for Bayern Munich in 2007.
Brief spells at Roma, Juventus and Genoa followed, with the Modenese forward linking up with former Italy coach Walter Zenga at Al Nasr earlier this year.
Winner of the FIFA World Cup™ in 2006, Toni has signed a one-year deal with Vincenzo Montella's new-look Fiorentina side.
Montella also managed to recruit Catania midfielder Cristian Llama on a one-year loan deal before the Italian transfer window closed.