Chelsea has paid £12 million for defender Mamadou Sarr, who was formerly with partner club Strasbourg.
The 19-year-old Frenchman has signed an eight-year contract and is eligible to compete in this month’s Club World Cup in the United States.
Enzo Maresca’s team begins their competition against LAFC on June 16, then faces Flamengo on June 20 and Tunisian champions ES Tunis on June 25.
Chelsea and Strasbourg agreed on a deal for Sarr in January, with the transfer expected to be completed this summer.
This season, he appeared in 28 matches across all competitions for Strasbourg. They finished seventh in Ligue 1 and earned a spot in the Conference League playoffs.
The signing is Chelsea’s fourth of the summer, following the arrival of Liam Delap for £30m from Ipswich Town, Estevao from Palmeiras for £29m and Dario Essugo from Sporting Lisbon for £18m.
Winger Jadon Sancho left Chelsea at the end of the season to return to Manchester United following his loan deal.