France international defender Axel Disasi has joined Chelsea from Monaco on a six-year contract for 45m euros (£38.5m).
Disasi, 25, offers defensive cover at Stamford Bridge after Wesley Fofana sustained a serious knee injury.
He played in all 38 of Monaco's Ligue 1 games last season.
"I hope to achieve very big things here. To win titles. I will do everything I can to achieve those objectives. I am very ambitious."
The Blues, meanwhile, have agreed a fee with Brighton for goalkeeper Robert Sanchez, while several bids for the Spaniard's team-mate Moises Caicedo have been rejected by the Seagulls.