Floyd Mayweather -- Justin Bieber Said Knock You Out!

Of course Floyd Mayweather took care of business last night against Miguel Cotto -- he had the dreaded masters of intimidation in his corner, like Lil Wayne, 50 Cent and ... Justin Bieber?

Bieber walked out with Mayweather before the fight and was in the ring with him after to celebrate the victory.

It's unclear how the two hooked up, though Bieber did once compare himself to Mayweather in an interview. Earlier in the day, Bieber tweeted, "cinco de mayo.... #MayweatherCotto yeah @FloydMayweather today is a BIG day."

So we have to ask ...