Torreira spent last season on loan with Spanish giants Atletico Madrid and was part of their squad that won the LaLiga title.
The Uruguayan midfielder made 19 appearances in his stay at the Wanda Metropolitano Stadium.
Having arrived at Arsenal ahead of the 2018-19 season from Sampdoria, Torreira featured 89 times for the Gunners playing a part in their FA Cup Final match-winning squad in 2020.
Torreira could have joined Eintracht Frankfurt after the German side made an offer to sign him but the 25-year-old's preference was a return to Italy.
Torreira will always be remembered at Arsenal with his iconic goal in a famous north London derby win during his first season.
The defensive midfielder was reportedly training alone at Colney since arriving back in London last week before finalising his move to Fiorentina.
Several Italian clubs, including AS Roma and Lazio, enquired about him but Fiorentina have been able to defeat their rivals for his signature.
Arsenal are yet to record a first Premier League win of the season after losing to Brentford and Chelsea.
Meanwhile Fiorentina after selling their captain German Pezzella to Real Betis also failed to win their first Serie A game of the season, losing 3-1 to Jose Mourinho’s AS Roma.
But with Torreira having a lot of experience in the Serie A will add more quality to the Fiorentina midfield.
