Germany winger Marin has made just six appearances for the Blues since arriving from Werder Bremen in 2012 but won the Europa League on loan at Sevilla last season.
The 25-year-old is not part of Jose Mourinho’s plans in West London and moves to Serie A for the coming campaign.
Fiorentina will have the option of making the loan permanent for £4.2million next summer.
Sevilla had reportedly wanted to take Marin back to Spain this season, while Hannover and Besiktas have both been linked with bids.
Written by Steve Milne
