Chelsea says Michael Essien has left the club to join AC Milan, ending the Ghana midfielder's 8½-year stint at Stamford Bridge.
The Premier League team made the announcement on Monday without disclosing any financial or contract details.
At one time one of the world's top midfielders, the 31-year-old Essien has lost his first-team place in recent years and spent last season on loan at Real Madrid.
Chelsea manager Jose Mourinho says "we would have been happy had he stayed because he is fantastic player for the team, but he's given everything for the club and a great chance arrived for his future."
Essien made 256 appearances for Chelsea after joining from Lyon in 2005.