After coming through Borussia Monchengladbach's ranks and moving to Werder Bremen in 2009, the winger's form was enough to convince Chelsea to part with an estimated €12 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.
The 16-cap Germany international spent last season on loan at Sevilla, chipping in with seven assists in 30 appearances, and hopes to have a similar impact at Fiorentina.
"I'm looking forward to an exciting time at Fiorentina and I will do my best to support the team," he wrote on Facebook. "I will learn Italian as fast as possible but for the time being I will still need the help of an interpreter.
"My special thanks go to Sevilla and to all the fans. It was a great experience to play in Spain and I had an amazing time with lots of fun."
Fiorentina get their Serie A campaign under way at Roma on Saturday, when Marin could come up against former Chelsea team-mate Ashley Cole.