Kosei Tanaka (田中 恒成, Tanaka Kōsei, born 15 June 1995) is a Japanese professional boxer. He is a four-weight world champion, having held the World Boxing Organization (WBO) mini-flyweight title from 2015 to 2016; the WBO junior-flyweight title from 2016 to 2017; the WBO flyweight title from 2018 to 2020; and the WBO super-flyweight title in February 2024. Upon winning his first world title in only his fifth bout, he became the fastest Japanese fighter ever to become a world champion. Along with Vasyl Lomachenko, he is also the fastest ever fighter to win titles in three weight-classes, having accomplished the feat in only 12 bouts. He also surpassed Oscar De La Hoya to become the fastest four division champion, setting the bar at 21 fights.