Chelsea have announced on their official website that Mason Mount has signed a contract until 2021.
The youngster will however spend next season on loan at Vitesse.
He joins Charlie Colkett, Fankaty Dabo and Mukhtar Ali at Vitas.
Mount was named Academy Player of the Year after helping Chelsea win the FA Youth Cup and the Under-18s title.
"Vitesse feels like me the next step in my career," Mount told the Dutch club's website.
"Vitesse gave me a good feeling when discussing [the move].
"I also talked to other players like Lewis Baker and Mukhtar Ali, who were all positive about the club."
