Chelsea have completed the signing of Benoit Badiashile from Monaco.
Badiashile has signed a seven-and-a-half-year contract after Chelsea paid a fee reportedly in the region of €38million (£33.7m).
The 21-year-old centre-back has been a regular in Monaco's defence since breaking into the first team in the 2018-19 season.
Chelsea were keen to improve their options in defence, with a left-sided player coveted following Antonio Rudiger's move to Real Madrid.
The Blues reportedly wanted to sign Josko Gvardiol, who starred for Croatia at the World Cup, but were unable to agree a fee with RB Leipzig.
Chelsea signed Wesley Fofana for £75m in August, but the former Leicester City defender has suffered several injury setbacks.
Badiashile, with 106 Ligue 1 appearances for Monaco to his name, brings plenty of experience as well as adding a youthful flavour to a defence that also includes Kalidou Koulibaly and veteran campaigner Thiago Silva.
He becomes Chelsea's second signing of the transfer window, after the arrival of David Datro Fofana, while Brazilian youngster Andrey Santos is also set to join.
"I'm very happy to be here at Chelsea," Badiashile told the club's official website. "I'm so excited to start playing for this club. I can't wait to see the fans and start playing in the best league in the world."
Chelsea co-owner and chairman Todd Boehly added: "We're delighted to welcome Benoit to Chelsea. 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.
"We are sure he will form an important part of our team in the years ahead."
Chelsea also want to bring in Enzo Fernandez, after the Benfica midfielder impressed for Argentina in Qatar.
A deal appeared likely earlier this week, but on Thursday Chelsea's approach was criticised by Benfica coach Roger Schmidt, who accused the Blues of showing a lack of respect in how they have gone about negotiations.