The 22-year-old, who played a leading role in Monaco's run to the Champions League semi-finals last season has signed a five-year contract at Stamford Bridge.
The France international has reportedly cost the Blues around £40million.
He told the club website: "I am very happy to be here and to join this great team
I have grown up watching Chelsea
Signing was the natural thing for me because it is a club I loved very much in my childhood."
Source : PA
Source: PA