At the end of 2014 he signed his first professional contract with French club Rennes. He made his Ligue 1 debut at just the age of 17 and he scored his first goal against Girondins. His progress attracted the attention of Borussia Dortmund and he signed for the Bundesliga club in the summer of 2016. Dembélé adapted quickly to German football, scoring 12 goals and providing 20 assists in his first season.