Justin Bieber -- Karaoke with Floyd Mayweather in Bora Bora (VIDEO)

Justin Bieber made a French Polynesian paradise that much better for a group of well-to-do vacationers ... by giving an impromptu performance during karaoke night.

Biebs has been vacationing in Bora Bora with Floyd Mayweather -- while Money celebrates new found retirement.

We're told the two stopped by their hotel's bar during karaoke night on Saturday ... when Justin decided to treat everyone to -- of course -- his own song, "What Do You Mean," before getting on piano and playing "Let It Be."

The 25 in attendance weren't exactly JB's target demo ... but enough Tahitian Sunrise cocktails can turn anybody into a fan.