Oleksandr Oleksandrovych Usyk (Ukrainian: Олександр Олександрович Усик, pronounced [olekˈsɑndr ˈusɪk]; born 17 January 1987) is a Ukrainian professional boxer. He has held the undisputed championship in two weight classes, at cruiserweight and heavyweight, and reigned as the undisputed world heavyweight champion from May to June 2024. He has also held the International Boxing Organization (IBO) title since 2021, and the Ring magazine title since 2022.

Usyk is the first undisputed heavyweight champion since Lennox Lewis was stripped of the title on 12 April 2000, and the first heavyweight in history to hold the world titles of all four major sanctioning bodies—the World Boxing Association (WBA) (Super version), World Boxing Council (WBC), International Boxing Federation (IBF), and World Boxing Organization (WBO)—in the "four-belt era". Previously he held the undisputed cruiserweight championship from 2018 to 2019, and is the first boxer to become the undisputed cruiserweight and heavyweight champion since Evander Holyfield in 1990. Usyk is also the third male boxer in history (after Terence Crawford and Naoya Inoue) to become the undisputed champion in two weight classes in the four-belt era.

As an amateur boxer, Usyk won heavyweight gold medals at the 2011 World Championships and 2012 Olympics while accumulating a record of 335 wins and 15 losses. He turned professional in 2013, winning his first regional title in 2014 after five fights and his first world title in 2016 after ten fights. By winning the undisputed cruiserweight championship in 2018, in his 15th professional fight, Usyk became the first Ukrainian undisputed champion in history. Three of his four titles were won during the inaugural World Boxing Super Series tournament, in which he won the Muhammad Ali Trophy, as well as the Ring and lineal titles. Usyk was named the 2018 Fighter of the Year by ESPN, The Ring and the Boxing Writers Association of America (BWAA).

He vacated his cruiserweight titles in 2019 to move up to heavyweight. In 2021, Usyk defeated unified heavyweight champion Anthony Joshua to win the WBA, IBF, WBO, and IBO titles. He defended the titles in a rematch against Joshua in 2022, while winning the vacant Ring title. In May 2024, Usyk defeated Tyson Fury to claim the WBC title and the undisputed championship in his second weight class.