Marin spent last season away from Stamford Bridge, on loan at UEFA Europa League winners Sevilla, and racked up seven assists in 30 games for the Spanish club.
That was not enough to convince Chelsea boss Jose Mourinho of Marin's talents, though, and he will join Vincenzo Montella's UEFA Champions League-chasing outfit.
The 16-cap Germany international is the second player to move to Serie A from Stamford Bridge following Ashley Cole's move to Roma.
Marin could even end up making his Fiorentina bow against his former colleague when Montella takes his side to the Stadio Olimpico on Serie A's opening day.
After coming through Borussia Monchengladbach's ranks and moving to Werder Bremen in 2009, Marin's form was enough to convince Chelsea to part with an estimated £10 million to secure his services in April 2012.
Injury prevented Marin from hitting the ground running at Stamford Bridge with subsequent struggles for game-time under Roberto Di Matteo and Rafael Benitez in 2012-13.
He will now look to impress for a Fiorentina side aiming to go one better than last season's fourth-place Serie A finish and reach the Champions League qualification spots.
