Ali, is reported to have died in a Phoenix-area hospital after being hospitalized this week for a respiratory ailment.
Former world heavyweight champion Muhammad Ali, whose record-setting boxing career, unprecedented flair for showmanship, and controversial stands made him one of the best-known figures of the 20th century, died on Friday aged 74, media reports said.
Family spokesman Bob Gunnell told NBC News that Ali died in a Phoenix-area hospital. Reuters could not immediately confirm the report.
"My heart is deeply saddened yet both appreciative and relieved that the greatest is now resting in the greatest place," boxer Roy Jones Jr. said on Twitter.
Ali was hospitalized this week for a respiratory ailment. The former prize fighter had long been suffering from Parkinson's syndrome, which impaired his speech and made the once-graceful athlete almost a prisoner in his own body.
Ali proclaimed himself "the greatest" - as well as "the boldest, the prettiest, the most superior, most scientific, most skillfullest."
Few could argue with him at his peak in the 1960s. With his dancing feet and quick fists, he could - as he put it - float like a butterfly and sting like a bee. He was the first person to win the heavyweight championship three times.
Ali became much more than a colorful and interesting athlete. He spoke boldly against racism in the '60s, as well as the Vietnam War.
Ali was born in Louisville, Kentucky, on Jan. 17, 1942, as Cassius Marcellus Clay Jr., a name shared with a 19th century slavery abolitionist. He later changed his name after his conversion to Islam.
Ali is survived by his wife, the former Lonnie Williams, who knew him when she was a child in Louisville, along with his nine children.