Charlton have made their seventh signing of the summer by landing defender Naby Sarr from Sporting Lisbon for an undisclosed fee.
The 21-year-old centre-half, a France Under-21 international, has agreed a five-year contract at The Valley.
"We needed another defender and Naby is the perfect fit," Addicks boss Guy Luzon told the club website.
"He is a player the club has been watching for a while and we nearly got him in January, but now we have done everything we can to get him this summer.
"He is strong, physical and another player who will do very well in the Championship."
Luzon has also signed Zakarya Bergdich, Patrick Bauer, El-Hadji Ba, Simon Makienok, Ahmed Kashi and Cristian Ceballos ahead of the new season.