The 18-year-old was seen escorting Mayweather, 35, to the boxing ring and also carried two of the boxer's championship belts, reported People magazine.
"Cinco de Mayo...Yeah Floyd Mayweather, today is a big day," he posted on Twitter.
Later, after Mayweather won a unanimous decision against Miguel Cotto, Bieber wrote on Twitter, "Hard work and dedication, congrats champ. Glad I could be a part of it."
Mayweather competed on the fifth season of Dancing With The Stars in 2007. His partner was Karina Smirnoff and the two were the fourth couple to be eliminated in the competition that season, landing in ninth place.
