Tiemoue Bakayoko has joined Chelsea from Monaco and become the club's second-most expensive signing of all time.
Chelsea have paid £40m for Tiemoue Bakayoko - their second-biggest signing
Bakayoko, an athletic central midfielder, has signed a five-year contract at Stamford Bridge and will join up with France international team-mate N'Golo Kante.
Tiemoue Bakayoko made 46 appearances for Monaco last season
Bakayoko said: "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.
"I am now looking forward to evolving under this fantastic coach (Antonio Conte) and alongside so many great team-mates."
Chelsea's technical director Michael Emenalo said: "Tiemoue was outstanding as Monaco emerged as one of Europe's best sides last season and we are delighted to welcome him to Chelsea.
Antonio Conte has added a third signing after Willy Cabellero and Antonio Rudiger
"Despite his young age he already has plenty of experience as a French title winner and having competed in the latter stages of the Champions League, we believe he will add considerable quality to our squad."
Bakayoko becomes Chelsea's third signing of the summer after the arrivals of back-up goalkeeper Willy Caballero and centre-back Antonio Rudiger from Roma.