The beloved 74-year-old sports hero, who had been battling Parkinson's disease for decades, passed away on Friday in a hospital in Phoenix, Arizona, where he had been admitted earlier this week suffering from respiratory problems, AFP reported.
"After a 32-year battle with Parkinson's disease, Muhammad Ali has passed away at the age of 74," Spokesman Bob Gunnell said.
"The three-time World Heavyweight Champion boxer died this evening."
Funeral arrangements for Ali would be announced Saturday, he added.
Concern for Ali had grown throughout Friday, and tributes immediately poured in for "The Greatest," who was known globally not only for his storied ring career but also for his humanitarian activism.
Ali had been hospitalized multiple times in recent years.
He spent time in hospital in 2014 after suffering a mild case of pneumonia and again in 2015 for a urinary tract infection.
His Parkinson's, thought to be linked to the thousands of punches he took during a brutal career studded by bruising battles inside the ropes, had limited his public speaking.
But he continued to make appearances and offer opinions through his family members and spokespeople.
In April, he attended a Celebrity Fight Night Dinner in Phoenix that raised funds for treatment of Parkinson's.
In December, he issued a statement rebuking US presidential hopeful Donald Trump's call for a ban on Muslims entering the United States.
"Muhammad Ali transformed this country and impacted the world with his spirit," said longtime boxing promoter Bob Arum.
"His legacy will be part of our history for all time."
His 30-year career, which stretched from 1960 to 1981 and saw him retire with a record of 56-5, included such historic bouts as the Rumble in the Jungle -- against George Foreman in 1974 in Kinshasa, Zaire.
Other defining moments of his career included two knockouts of Sonny Liston and his thrilling rivalry with Joe Frazier -- which saw the two men slug it out in the ring and verbally spar out of it.
"Ali, Frazier & Foreman, we were 1 guy. A part of me slipped away -- the greatest piece" Foreman wrote on Twitter shortly after Ali's death was announced.
Former heavyweight champion Mike Tyson added: "God came for his champion. So long great one. @MuhammadAli #TheGreatest #RIP."
Ali, born Cassius Marcellus Clay Jr in Louisville, Kentucky, dazzled fans with slick moves in the ring, and with his wit and engaging persona outside it.
His refusal to serve in the Vietnam War saw him banned from the sport for years, but the US Supreme Court overturned his conviction for draft dodging in 1971.
He took the name of Muhammad Ali after converting to Islam in 1964, soon after he had stunned the sport by claiming the title with a monumental upset of Sonny Liston.
He was vilified in some quarters for that conversion and his outspoken stance on Vietnam and civil rights issues, but he held firm to his beliefs and eventually earned accolades as an activist.
He was chosen to light the Olympic torch in 1996 in Atlanta and was named a UN messenger of peace in 1998.
He received the highest US civilian honor, the Presidential Medal of Freedom, in 2005.
By Tasnim News Agency
