Chelsea signed former Nice defender Malang Sarr on a free transfer on Tuesday.
Sarr left Nice when his contract expired in the close season and agreed a five-year deal with the Premier League club.
The 21-year-old centre-back is seen as one for the future by Chelsea manager Frank Lampard and will be sent on loan this coming season to build his experience.
Nice-born Sarr racked up more than 100 appearances at his hometown club since his senior debut aged just 17.
He is eligible to play for Senegal due his parents, but he has represented France from Under-16 to Under-21 levels and captained their Under-18s.
"The opportunity to sign Malang was one we could not miss," Chelsea director Marina Granovskaia told the club's website.
"He is a tremendous prospect and we will be monitoring him closely during his loan period, hoping he will soon be back at Chelsea.
"We are very happy to welcome him to the club today."
Sarr is Chelsea's latest signing in a busy close season that has seen the arrivals of Timo Werner, Hakim Ziyech and Ben Chilwell.
The Blues are also reportedly close to signing former Paris Saint-Germain defender Thiago Silva on a free transfer.