Floyd Mayweather says he is mourning the loss of "one of the most important people in my life" after the death of his uncle and former trainer Roger.
Roger Mayweather died on Tuesday, aged 58, following years of ill health.
After being a world champion himself, he helped train his nephew to become one of the greatest boxers in history over an unbeaten 50-fight career.
"Roger was a great champion and one of the best trainers in boxing," said Floyd Mayweather, 43.
"My uncle was one of the most important people in my life inside and outside of the ring," added Mayweather, who was already grieving after the mother of his three children died on 11 March.
"Unfortunately, his health was failing him for several years and now he can finally rest in peace. Roger meant the world to me, my father Floyd Sr, my uncle Jeff, our whole family, everyone in and around the Mayweather Boxing Gym and the entire boxing world. It is a terrible loss for all of us."
A statement from Mayweather Promotions added: "Roger's spirit lives on with us forever."
No cause of death has been confirmed.
Roger Mayweather held world titles at featherweight and welterweight in the 1980s.
As a trainer, he guided his nephew to wins over Ricky Hatton and Oscar de la Hoya.