The Viola have been looking to bolster their midfield ranks following the departures of Borja Valero and Matias Vecino to Inter Milan.
Benassi joins after a three-year spell with Torino which saw him claim the captain's armband last season, while he is also an Italy under-21 international.
Fiorentina confirmed the deal via their official website and it's understood they parted with 10million Euros plus bonuses for his services.
A statement on the club's website said: "ACF Fiorentina is pleased to announce the signing of Marco Benassi from Torino FC on a permanent transfer.
"The Italy U21 midfielder, who is expected in Florence later today, will undergo his medical on Thursday morning before putting pen to paper on a five-year contract."