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Floyd Mayweather's uncle, Roger Mayweather -- one of the most famous boxing trainers ever -- has died, Floyd's camp confirms with TMZ Sports. He was 58.

Roger was a KEY member of Floyd's training team -- and sculpted him into becoming the global phenom he is today. Many considered Roger the best trainer in the world.

He was also a pretty good boxer and battled with some pretty big stars, including Julio César Chávez and Ray "Boom Boom" Mancini.

He was also a 2-weight world champion -- holding the WBA and lineal super featherweight titles back in the day, along with the WBC light welterweight title (from '87 to '89).

Roger had been in declining health for years. In fact, Floyd opened up about Roger's health back in 2015 and blamed boxing for damaging his brain.

"My uncle Roger Mayweather has lost a lot of memory from the sport of boxing," Floyd said ... "He's only in his 50s, but it seems like he's an old man in his 80s."

A few years ago, Roger went missing -- and fortunately, he was located by law enforcement officials.

We're told Floyd was close with Roger and is deeply emotional about his passing.