The Toffees confirmed the news via Twitter on Saturday:
Fiorentina welcomed his arrival in a statement on their website, noting they have the option to purchase the player outright.
Mirallas has been with Everton since 2012, but the Belgian's role has decreased in recent years, and he spent part of last season on loan with former club Olympiacos.
The 30-year-old was never expected to play a major role for the Toffees this year even if he had stayed, although fans noted he performed well in pre-season:
A youth product of Standard Liege's vaunted academy, Mirallas moved to France early in his professional career before joining Everton via Olympiacos. He settled in nicely at Goodison Park and enjoyed his best Premier League season in the 2013-14 campaign, with nine goals and seven assists.
The Belgium international signed a new contract last year, but since then, the club has gone through some major changes. In Marco Silva and Marcel Brands, the Toffees have hired a new manager and director of football, and it would seem neither believes Mirallas has a place in their side.
As a mobile and versatile forward who can play in a central role or out wide, Serie A should suit Mirallas well. His profile is similar to that of former Liverpool man Suso, who has found plenty of success since moving to AC Milan.
Fiorentina relied heavily on the young duo of Federico Chiesa and Giovanni Simeone last year, and it's not a bad idea to add a more experienced option to the squad to help guide those prospects. Mirallas should be excellent as a depth option thanks to his versatility.
Cyril Thereau will likely be his main competitor for a starting position, and Mirallas will like his chances of beating the Frenchman.