Two-times welterweight world champion Jose "Mantequilla" Napoles died on Friday in Mexico City aged 79, the World Boxing Council announced on its website.
Cuba-born Napoles forged a Hall of Fame career, with a record of 81-7, including 54 knockouts.
He was granted Mexican citizenship in 1969 after beating Texan Curtis Cokes in a world title fight.
Napoles` final bout was a losing one in front of 40,000 fans against Briton John H Stracey, who paid tribute to his former rival on Friday.
"I`m so very proud to have shared a ring with the great Jose Napoles," Stracey said.
"What a magnificent fighter he was and what a great champion. Look how long he reigned as a champion. This is such a sad day."