Oleksandr Oleksandrovych Usyk (/ˈʊsik/; OO-seek; Ukrainian: Олекса́ндр Олекса́ндрович У́сик; born 17 January 1987) is a Ukrainian professional boxer. He reigned as the undisputed cruiserweight champion from 2018 until he vacated his titles in 2019, being the first cruiserweight in history to hold all four major world championships—the WBA (Super), WBC, IBF and WBO titles.

As an amateur, Usyk won gold medals at the 2011 World Championships and 2012 Olympics, both in the heavyweight division. He turned professional in 2013 and won the undisputed cruiserweight championship in his 15th professional fight in 2018. Three of his titles were won during the inaugural World Boxing Super Series, in which he won the Muhammad Ali Trophy, as well as The Ring magazine and lineal cruiserweight titles. For his achievements, Usyk was named the 2018 Fighter of the Year by Sports Illustrated, ESPN, The Ring, and the Boxing Writers Association of America.

Usyk is the most recent male boxer to simultaneously hold the WBA, WBC, IBF and WBO titles since Terence Crawford in 2017, and only the fifth boxer in history to do so after Crawford, Jermain Taylor, Bernard Hopkins, and Cecilia Brækhus. He is the first Ukrainian undisputed champion. He is particularly notable for the speed of his accomplishments, winning his first world title in his tenth fight, and becoming the undefeated champion of his weight class by his fifteenth fight. Usyk vacated his cruiserweight titles in 2019 to move up to heavyweight. By that point, in sixteen fights he had defeated five current or former world champions.

As of August 2020, Usyk is ranked as the world's fifth best active boxer, pound for pound, by the Transnational Boxing Rankings Board and The Ring, and sixth by ESPN, as well as the world's fifth best active heavyweight by BoxRec and tenth by ESPN.