It was announced on 8 November 2015 that Diakhaby signed for Rennes on a one–year contract. He was quickly assigned to the Rennes B for the rest of the 2015–16 season. While developing, the club's B team Christophe Gadby said about Diakhaby, saying: "He was well above the level. I remember a young man who was respectful of people and of instructions." It was announced on 4 July 2016 that Diakhaby signed his first professional contract with the club and was promoted to the first team ahead of the 2016–17 season.
