England Under-21 international Mason Mount has signed a five-year contract with Chelsea.
The 20-year-old midfielder spent last season playing under new Blues boss Frank Lampard during a loan spell at Derby.
"It's a massively proud moment for myself and my family," Mount told the club's website.
Mount has yet to make his debut for Chelsea's first team but impressed for Championship side Derby during the last campaign, scoring 11 goals in 44 appearances in all competitions.
He played against the Blues in a Carabao Cup tie in October.
Mount added: "I joined this club at six years old and it was always my goal to play for the first team one day.
"I'm really looking forward to the coming season now and working hard to have an impact here at Chelsea.
"I've been at the club for a long time already and hopefully I'll stay for a long time to come."
Mount had a loan spell at Vitesse Arnhem in 2017-18, scoring nine goals in 29 Eredivisie appearances and five more in three play-off games as the Dutch club qualified for the Europa League.
Chelsea director Marina Granovskaia said: "Mason has come right the way through the Chelsea academy and has always stood out as a fantastic talent and dedicated individual.
"We have monitored his development extremely closely while he has been out on loan and believe he is now ready to be a member of our squad competing at the top of the Premier League.
"Like another of our academy graduates, Ruben Loftus-Cheek, we are delighted Mason has committed himself to the club this summer and look forward to an exciting future ahead."