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.
Leon's health had been in decline recently ... the former boxing champ courageously battled prostate cancer for 5 years.
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.
The Ali fight was the most memorable moment of Leon's career ... but definitely not the only highlight.
In 1976, Spinks won a gold medal as a light heavyweight fighter in the Summer Olympics in Montreal.
Spinks -- born in St. Louis in 1953 -- developed a passion for boxing while in the U.S. Marines.
Pugilism ran deep in the Spinks family ... Leon was the older brother of boxer Michael Spinks -- a former 2-weight class world champion, and Olympic gold medalist (both brothers won gold in '76).
Leon was inducted into the Nevada Boxing Hall of Fame in 2017.