The 22-year-old has fallen down the pecking order at the London Stadium following the club's summer spending spree and heads to Florence in search of regular football.
The Swiss midfielder joined West Ham from Sion in his homeland in 2016 and has made 48 appearances for the club to date.
Stefano Pioli's 'La Viola' side are looking to improve on last season's showing as they finished eighth in the Italian top flight and face Sampdoria in their first game of the new season on Saturday.
Fiorentina are believed to have an option to make the deal permanent at any point during the loan.
13 August 2018