Born in Paris on 14 October 2004, Kassoum Ouattara began playing football at an early age, at US Ivry. He then moved to FC Montrouge 92 in 2017, before joining professional club Amiens in 2020. The left-back completed his training there and made his Ligue 2 debut against SC Bastia in August 2022. He was just 17 years and 310 days old at the time. He played a total of 11 games that season, then became a regular in his position at the start of the following season, where he played in 10 matches, all of them in the starting XI. Kassoum put in a number of fine performances and was a good centre-half, making his debut for the French national youth team (2 at U18 level, 3 at U19 level and 1 at U20 level) alongside other Monegasque players such as Edan Diop, Eliesse Ben Seghir and Malamine Efekele. Kassoum finally joined AS Monaco in November 2023 on a five-year deal. In his first season with the Red and Whites, he played 17 games and scored his first goal against Montpellier. Following on from a promising first season, the Amiens-trained player made 14 appearances for the red-and-whites, including 2 in the Champions League.