The pacy Remy, 27, arrives at Stamford Bridge after Chelsea activated a 10.5 million pound buyout clause ($17.42 million) in the player's contract.
"I feel very happy and very proud. When I heard Chelsea wanted to sign me I said "let's go" straight away because they are one of the best clubs in the world," Remy, who has scored five goals in 24 games for France, told the club's official website (www.chelseafc.com).
"I remember the amazing atmosphere at Stamford Bridge when I played there for Marseille and I can't wait to play for the first time in front of my new fans."
Chelsea moved for Remy after AC Milan agreed a two-year loan deal for misfit striker Fernando Torres.
Remy spent last season on loan at Newcastle United scoring 14 goals in 26 Premier League appearances following QPR's relegation from the Premier league.
He becomes the sixth signing for Chelsea, who have won their opening three league games to top the Premier League table.