Oleksandr Oleksandrovych Usyk (Ukrainian: Олександр Олександрович Усик, pronounced [olekˈsɑndr ˈusɪk]; born 17 January 1987) is a Ukrainian professional boxer. He has held the unified heavyweight championship since 2021, and the Ring magazine title since 2022. He held the undisputed championship in two weight classes—cruiserweight and heavyweight—and is the third male boxer in history (after Terence Crawford and Naoya Inoue) to become a two-weight undisputed champion in the "four-belt era".

As an amateur, 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 world title in 2016. By winning the undisputed cruiserweight championship in 2018 at the conclusion of the World Boxing Super Series tournament, Usyk became the first Ukrainian undisputed champion in history. In 2021, Usyk defeated unified heavyweight champion Anthony Joshua to win the WBA, IBF, and WBO titles. In May 2024, Usyk defeated Tyson Fury to claim the WBC title and the undisputed championship in his second weight class.

Usyk was the first undisputed heavyweight champion since Lennox Lewis was stripped of the title on 12 April 2000, and is the first and only cruiserweight and heavyweight boxer in history to hold the world titles of all four major sanctioning bodies—the World Boxing Association (WBA) (Super title), World Boxing Council (WBC), International Boxing Federation (IBF), and World Boxing Organization (WBO)—in the four-belt era. He previously held the undisputed cruiserweight championship from 2018 to 2019, and is the first boxer to become the undisputed champion in these two weight classes since Evander Holyfield in 1990. The Ring and the Boxing Writers Association of America named Usyk their Fighter of the Year in 2018 and 2024.

In February 2025, Sportico ranked Usyk at No. 7 among the highest-paid athletes of 2024, with estimated income $122 million. In May 2025, Forbes put him at No. 11 in its ranking of the highest-paid athletes in the world, with his earnings being estimated at $101 million.