Swiss international Edimilson joined West Ham from FC Sion in 2016 and has struggled to make an impact due to poor form and injuries, managing 48 appearances in all competitions.
Fiorentina reportedly paid an initial loan fee of around 900,000 pounds ($1.15 million) for the 22-year-old with the option to sign him permanently next season.
Fiorentina are looking to improve on last season's eighth-place finish in the Italian top flight.
Competition for places at West Ham has increased this season with new boss Manuel Pellegrini bringing in nine new players for about 100 million pounds.
West Ham, who have endured relegation battles in their previous two campaigns, began their Premier League season with a 4-0 defeat at Liverpool. They host Bournemouth on Saturday.
(Reporting by Shrivathsa Sridhar in Bengaluru, editing by Ed Osmond)
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