Burton, a native of Flint, Mich., had been in and out of the hospital for the past year, but was never officially diagnosed with any illness. He was 78.
Carl Weathers, who played Apollo Creed in Rocky, also confirmed the news via Twitter:
In high school, Burton was a top-tier football player, as well as a pitcher for the school's baseball team, but pursued a boxing career after graduating. He won two light heavyweight titles in Flint in 1955 and '57, and then fought professionally in 1958 and '59.
After his boxing career stalled, Burton served three-and-a-half years in prison after being convicted of robbery. When he got out, he decided to become an actor.
Burton appeared in the first six Rocky films, but his poor health prevented him from even seeing the latest one, Creed, which was released last year. In addition to his role in the Rocky franchise, Burton's other credits include The Shining and Stir Crazy, as well as TV shows including NYPD Blue and Twin Peaks.
Burton's son, Martin, died of a heart attack in 2014 at age 43.