A year after failing to land a controlling stake in AC Milan, Italian-born Commisso has agreed to buy one of their top-flight rivals from the Della Valle family.
The 69-year-old, who is also the chairman of the New York Cosmos, announced the deal – worth a reported €160million – on Thursday to usher in a new era at the Stadio Artemio Franchi.
"I am forever a fan of Italian football and there are no words to describe how incredibly honoured I am to have the opportunity to contribute to write the next chapter in the story of a legendary club like Fiorentina," Commisso, chairman and CEO of Mediacom, said in a statement.
"Florence is known across the entire world as a city that represents the best of Italian culture. In these three years of contact to acquire the club, I've developed a deep understanding of how important La Viola is for this city and its fans.
"I would like to thank the Della Valle family for managing Fiorentina for the past 17 years. Diego and Andrea deserve great honours for saving this club from financial instability. They leave a strong foundation to build the club on."
Fiorentina finished 16th in Serie A last season and have not won any major honours since lifting the Coppa Italia in 2001.
However, they made it to the semi-finals of the competition this year, thumping Roma 7-1 in the quarter-finals en route, with Federico Chiesa grabbing a hat-trick, while Luis Muriel, Marco Benassi and Giovanni Simeone were also on the scoresheet.
They fell just short against Atalanta in the last four by a 5-4 scoreline over the two legs, losing the second game 2-1 after a thrilling 3-3 draw at home in the first encounter.