The 24-year-old arrived at Stamford Bridge last summer, during Roberto Di Matteo's tenure, from Werder Bremen for a fee of around £7m.
Compatriot Andre Schurrle's recent arrival from Bayer Leverkusen means Marin has fallen further down the pecking order and he has agreed to move to southern Spain to link up with Sevilla, who finished ninth in the 2012/13 La Liga table.
After moving to west London he quickly suffered a troublesome hamstring injury and had to wait to regain full fitness to make his competitive debut in the 6-0 Capital One Cup home victory over Wolverhampton Wanderers.
The German waited even longer to make his first league start for the Blues, with the home derby against Queens Park Rangers in early January ending in a surprise 1-0 defeat.
His first competitive goal for Chelsea followed in February's 4-1 win over Wigan Athletic and he went on to make 16 appearances in total last season.
New boss Jose Mourinho is looking to build a squad to his own requirements and Schurrle was effectively the Portuguese's first acquisition since returning to the Bridge.
More signings are expected following the departures of Ross Turnbull and Hilario and, with Belgian Thibault Courtois on loan at Atletico Madrid for a third successive season, a back-up goalkeeper is required as cover for Petr Cech.
With only youngster Jamal Blackman in place, Chelsea have been linked with Norwich City's England goalkeeper John Ruddy, who spent much of last season out injured with a thigh problem.