Valero, 35, was let go by Inter when his contract expired in June and had interest from Parma and Genoa.
However, he could not turn down a return to Florence, where he was a big fan favourite between 2012 and 2017.
Madrid-born Valero will wear the number 6 shirt at Fiorentina.
In his previous spell with the Viola, Valero made 212 appearances with a return of 17 goals and 45 assists.
B O R J A V A L E R O 💜
— ACF Fiorentina (@acffiorentina) September 16, 2020