Muhammad Ali (; born Cassius Marcellus Clay Jr.; January 17, 1942 – June 3, 2016) was an American professional boxer and activist. Nicknamed "the Greatest", he is regarded as one of the most significant sports figures of the 20th century and is often regarded as the greatest heavyweight boxer of all time. He held the Ring magazine heavyweight title from 1964 to 1970. He was the undisputed champion from 1974 to 1978 and the WBA and Ring heavyweight champion from 1978 to 1979. In 1999, he was named Sportsman of the Century by Sports Illustrated and the Sports Personality of the Century by the BBC.

Born and raised in Louisville, Kentucky, he began training as an amateur boxer at age 12. At 18, he won a gold medal in the light heavyweight division at the 1960 Summer Olympics and turned professional later that year. He converted to Islam after 1961. He won the world heavyweight championship, defeating Sonny Liston in a major upset on February 25, 1964, at age 22. During that year, he denounced his birth name as a "slave name" and formally changed his name to Muhammad Ali. In 1967, Ali refused to be drafted into the military, owing to his religious beliefs and ethical opposition to the Vietnam War, and was found guilty of draft evasion and stripped of his boxing titles.