The out-of-favor 25-year-old will spend the season with Vincenzo Montella in Florence after helping Sevilla win the Europa League last season.
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Marin moved to Stamford Bridge in the summer of 2012 but failed to break into the starting XI, making just 16 appearances in all competitions in his debut season in England.
The 25-year-old spent last season on loan at Sevilla, making 28 appearances in all competitions without scoring, though he was part of the squad which won the Europa League.
The German's arrival gives Fiorentina boss Vincenzo Montella more options in attacking midfield, though the Italian on Monday reiterated that Manchester United and Barcelona target Juan Cuadrado will not be sold.