Tiemoue Bakayoko has joined Chelsea from Monaco and become the club's second-most expensive signing of all time.
Sky Sports News HQ understands the 22-year-old has cost the Blues £40m - some £10m less than Fernando Torres in 2011.
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.
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.
"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.
The Blues are also in the hunt for a new striker, having decided to listen to offers for Diego Costa and Nemanja Matic, who were both left out of the pre-season tour of the Far East.