Pioli returns to Italy after a short spell in charge of Al-Nassr in Saudi Arabia.
He has signed a deal with the Viola to 2028.
Fiorentina announced today: "ACF Fiorentina announces that Stefano Pioli is the new Viola first-team coach.
"Pioli, who returns to Florence after wearing the purple jersey as a player between 1989 and 1995, and leading the club as coach from 2017 to 2019, has signed a contract that will bind him to the club until June 30, 2028."
Pioli left AC Milan a year ago, having led the Rossonero to the Scudetto in 2022.