Switzerland international Fernandes joined the Hammers from Sion two years ago and has played 48 times for the club in all competitions to date.
His 2017/18 season was massively disrupted by injury, while the summer arrivals of Jack Wilshere, Carlos Sanchez, Felipe Anderson and Andriy Yarmolenko has significantly increased competition for places in the variety of roles that Fernandes can play fill.
A short statement from his parent club concluded read: "The 22-year-old Swiss midfielder links up with Stefano Pioli’s men as they look to build on an eighth-place finish in the Italian top flight last season.
"West Ham United wishes Edimilson all the best for his spell in Italy."
Fernandes made 14 Premier League appearances for the Hammers last season, demonstrating great versatility to first Slaven Bilic, and then David Moyes, by filling in at right-wing back, despite his favoured position central midfield.
The 22-year-old will now be hoping to establish himself as a regular starter with Fiorentina, who finished last season eighth in Serie A and will kick off their 2018/19 campaign against Sampdoria in Genoa this coming Saturday.