According to RMC Sport, the 28-year-old will sign a five-year contract with Chelsea pending an agreement between the clubs.
Rennes want €26m for Senegalese international Mendy, who has told the club that he wishes to join Chelsea.
Chelsea have offered €20m and talks are ongoing with the two clubs trying to reach an understanding.
Kepa has failed to impress since his arrival from Athletic Bilbao and Mendy will provide stern competition.
Mendy kept 13 clean sheets and conceded 31 goals in 33 appearances for Rennes last season.