At age 14, Doumbouya moved to the renowned French sports institute INSEP in Paris and joined its affiliated club Centre Fédéral de Basket-ball, a member of the Nationale Masculine 1 (NM1), the amateur third-tier division of French basketball. He made his 2015–16 season debut on 26 September 2015, recording 12 points, five rebounds, and three steals against Aurore de Vitré. In turn, Doumbouya became the first player born in the 2000s to play in the top three French leagues. On 8 March 2016, he scored a season-high 23 points in a loss to Vendée Challans. Doumbouya finished the season averaging 10.6 points, 3.3 rebounds, 1.2 assists, and 1.4 steals per game.
