Kalidou Koulibaly was born on 20 June 1991 in Saint-Dié-des-Vosges, France, to Senegalese parents. He began his footballing journey with local club SR Saint-Dié before joining the Metz academy. After being released as a teenager, he worked his way back and signed his first professional contract with Metz ahead of the 2010–11 season, going on to make 19 appearances in his debut campaign.
In 2012 he moved to Belgian club Genk, where he won the Belgian Cup in his first season. His performances earned him a move to Napoli in 2014 for around £6.5 million. During eight years in Naples he became one of Serie A's most reliable centre-backs, winning the Supercoppa Italiana in 2014 — scoring the decisive penalty in the shootout against Juventus — and the Coppa Italia in 2020. He made over 300 appearances for the club across all competitions.
A commanding, physically strong centre-back who reads the game well and is composed in possession, Koulibaly joined Chelsea in 2022. Although his time in west London has not brought silverware to date, he remains a senior figure in the squad.
Born in France, Koulibaly initially represented France at under-20 level before committing to Senegal. He made his senior international debut in 2015 and has since become a central figure for the Lions of Teranga, helping them reach the 2019 Africa Cup of Nations final and winning the tournament outright in both 2021 and 2022, defeating Egypt and Morocco respectively in the finals.