Chelsea signed France international defender Benoit Badiashile from Ligue 1 side Monaco on Thursday for a fee believed to be just under 40 million euros ($42 million).
The 21-year-old central defender signed a "seven-and-a-half-year contract", according to a statement from Chelsea.
Badiashile has been a pivotal figure at Monaco, helping the club finish third in Ligue 1 for the past two seasons.
He was handed his international debut by Didier Deschamps last September, although he did not make the World Cup squad.
"At 6ft 4in, Badiashile is a dominant defender in the air but equally comfortable with the ball at his feet, capable of playing short passes out from the back or switching the play with longer diagonals," read the Chelsea statement.
"His blistering speed is a key attribute and he has experience in both a back four and back three at club level."
Badiashile said he could not wait to "start playing in the best league in the world."
The young Frenchman could partner the vastly more experienced Senegalese international Kalidou Koulibaly, who Chelsea signed for a reported fee of £33 million ($38 million) last July.
Chelsea chairman Todd Boehly and co-controlling owner Behdad Eghbali said they were delighted to have secured the signature of one of Europe's most promising defenders.
"He's an excellent young player who has quickly established a strong reputation for himself and has a huge amount of experience for his young age," they said in a joint statement.
"We are sure he will form an important part of our team in the years ahead."
Badiashile will not be involved in Chelsea's clash at home to Manchester City on Thursday as they try and make up ground on the top four.
Chelsea are 10 points off the Champions League places following a New Year's Day draw at Nottingham Forest and have won only one of their last seven matches.