Benjamin Mendy has signed a five-year contract with Monaco after opting to leave Marseille.
The 21-year-old, who spent three years at the Stade Velodrome, started his career with Le Havre and has represented France at every age group up to the under-21s.
"I am very proud to get involved with AS Monaco," Mendy told the club's official website. "It's a new challenge for me. I hope to progress...with this great club."
Mendy made 24 Ligue 1 appearances for Marseille last season, scoring once, as they finished 13th.
Leonardo Jardim's Monaco took third to secure a spot in the third qualifying round for the Champions League.