Muhammad Ali died from septic shock and went peacefully, a spokesman for the family said.
He'd been in hospital in Arizona for a few days, with his family gathered around. They had a full day to say their final goodbyes to the 74 year-old.
Ali's family have this morning announced that his funeral will span two days, beginning with a private family service on Thursday, US time.
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Family spokesman Bob Gunnell says an Imam will lead the public service the following day in his hometown in Kentucky.
The service - which starts at two in the afternoon - will feature eulogies from former president Bill Clinton, Billy Crystal and Bryant Gumbel.
For those unable to attend the funeral service, it will be streamed live from www.alicenter.org, his family said in a statement.
The event will be open to the public with limited seating. Information and details on tickets would be released as they become available.
A procession is planned through Louisville starting in the morning to allow the general public to pay their last respects to the late boxing champ.
Meanwhile, Prime Minister John Key has tweeted that he once met Ali.
A legend has passed. I had the honour of meeting Muhammed Ali in LA in 1983. A real privilege.— John Key (@johnkeypm) June 4, 2016
The mad butcher Sir Peter Leitch met Mohammad Ali when the champ visited New Zealand in 1979.
He said it was one of his greatest moments.
Sir Peter's mate was a police photographer taking photos of personalities at the time.
A photo was snapped of Sir Peter and Ali which was signed by the boxer.
Sir Peter said it was one of his most prized possessions.
New Zealand broadcaster Pete Montgomery holds fond memories of Ali, from their meeting in 1979.
He was filling in on a TV2 show and had an interview arranged with Ali and Sir Bob Jones.
Montgomery says Ali was only probably expecting to be in the studio for a few minutes, but in the end the three men talked for an hour and a half.
He says he could feel the boxer easing up as the interview went on, and his wonderful humour and quick responses came through.
He rates Muhammad Ali on what he's calling a short list of the most outstanding sports people of all time.