Muhammad Ali (born Cassius Marcellus Clay Jr.; January 17, 1942 – June 3, 2016) was an American professional boxer, activist, showman, and philanthropist. Nicknamed "The Greatest," he is widely regarded as one of the most significant and celebrated figures of the 20th century and one of the greatest boxers of all time.

Ali was 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 the same year. After 1961, he converted to Islam. On February 25, 1964, at age 22, he sensationally took the world heavyweight championship title from Sonny Liston. On March 6, 1964, he announced that he would no longer be known as Cassius Clay, but as Muhammad Ali. In 1966, Ali refused induction into the army, citing his religious beliefs and ethical opposition to the Vietnam War. He was found guilty of draft evasion, faced five years in prison, and was stripped of his boxing titles. He remained free while he appealed the decision to the Supreme Court, which overturned his conviction in 1971, but he did not enter the ring for nearly four years and lost his athletic prime. Ali's actions as a conscientious objector to the Vietnam War made him an icon for a broader counterculture generation, as well as a highly visible figure of racial pride for African Americans during the civil rights movement and throughout his career. As a Muslim, Ali was initially associated with Elijah Muhammad's Nation of Islam (NOI). He later disavowed the NOI, adhering to Sunni Islam and supporting racial integration, as did his former mentor Malcolm X.

Ali was the leading heavyweight boxer of the 20th century and remains the only three-time lineal champion of that division. His combined records — victories over 21 boxers for the world heavyweight championship and 14 wins in unification title fights — stood for 35 years. He is the only fighter whom BoxRec named the world's best heavyweight twelve times. He was ranked in BoxRec's top ten heavyweights seventeen times — the third most in history. He won 8 fights rated 5 stars by BoxRec — the third most in heavyweight history. Ali is the only boxer named "Fighter of the Year" by The Ring magazine six times. He was recognized as the greatest heavyweight of all time, as well as the greatest athlete of the 20th century by Sports Illustrated, "Sports Personality of the Century" by the BBC, and the third greatest athlete of the 20th century by ESPN SportsCentury. He took part in several historic boxing matches and conflicts, most notably his fights with Joe Frazier, such as the "Fight of the Century" and the "Thrilla in Manila," as well as his fight with George Foreman, known as the "Rumble in the Jungle," which was called "possibly the greatest sporting event of the 20th century" and was watched by a record estimated television audience of 1 billion viewers worldwide, making it the most-watched live television broadcast of its time. Ali thrived in the spotlight at a time when many fighters let their managers speak for them, and he was often provocative and eccentric. He was known for his verbal jabs and often improvised with rhymes and spoken-word poetry, anticipating elements of hip-hop.