The former world heavyweight boxing champion, one of the world's best-known sportsmen, has passed away at a hospital in the US city of Phoenix in Arizona.
He had been fighting a respiratory illness, a condition that has been further complicated by Parkinson's disease.
Born Cassius Marcellus Clay and nicknamed "The Greatest", Ali shot to fame by winning light-heavyweight gold at the 1960 Rome Olympics.
He eventually retired in 1981, having won 56 of his 61 fights.
Crowned "Sportsman of the Century", Ali was noted for his pre- and post-fight talk and bold fight predictions just as much as his boxing skills inside the ring.
But he was also a civil rights campaigner and poet who transcended the bounds of sport, race and nationality.
Asked how he would like to be remembered, he once said: "As a man who never sold out his people. But if that's too much, then just a good boxer. I won't even mind if you don't mention how pretty I was."
Muhammad Ali is still in hospital two days after being admitted with a respiratory issue.
There has been no official update on Ali's condition and concern has grown throughout yesterday.
An enormous number of goodwill messages for the 74-year-old, who was diagnosed with Parkinson's Disease in 1984, have been posted on social media.