On 28 April 2012, Marin agreed a deal with Chelsea. He later revealed that he joined Chelsea for the prospect of a new challenge, indicating that moving to London was a dream come true and he would officially join the Blues as their first summer signing once the transfer window officially opened on 1 July. He signed a five-year deal with Chelsea, keeping him at the club until 2017. Marin was given the number 21 shirt, previously worn by Salomon Kalou, who left Chelsea for Lille OSC.
Marin made his debut for Chelsea in a pre-season friendly against Major League Soccer (MLS) side Seattle Sounders FC, scoring Chelsea's third goal in a 42 win. Marin missed the Community Shield with a hamstring injury. He also missed Chelsea's first game of the season against Wigan Athletic with the hamstring injury that kept him out of FA Community Shield against Manchester City a week earlier.
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