Leon Spinks -- former undisputed heavyweight boxing champ -- who defeated Muhammad Ali in 1978 ... has died after a 5 year battle with cancer.
TMZ Sports has confirmed Spinks passed away Friday evening at the age of 67 ... surrounded by his wife Brenda and several other close friends and family.
Spinks was the victor in one of the biggest upsets in sports history ... when he shocked the world by defeating Ali in a 15-round, split decision win in Las Vegas in February 1978.
In defeating Ali, Spinks -- the new heavyweight champ -- became the only fighter to ever take a belt from The Greatest.