Iachini returns to the club just four months after he was dismissed following an underwhelming start to the 2020/2021 season.
The 56-year-old left in November, with Fiorentina 12th on the Serie A table but returns as La Viola have dropped to 14th position, just seven points above the relegation zone.
Iachini’s previous spell as Fiorentina manager lasted from December 2019 until his dismissal, and he led the club to 12 wins from 31 league games.
A statement from Fiorentina read: “ACF Fiorentina announce that they have entrusted the technical leadership of the First Team to Beppe (Giuseppe) Iachini.
“The coach will direct his first training session at the Davide Astori Sports Centre and will resume contact with the team today.”
Some of Iachini’s previous clubs include Sassuolo, Udinese, Sampdoria and Palermo.
Fiorentina, who have just three wins in their last 10 games, visit Genoa in their next league outing after the international break.