Justin Bieber -- Floyd Mayweather's Hype Man At Vegas Boxing Match

Justin Bieber has successfully weaseled his way into Floyd Mayweather's inner circle -- AGAIN -- TMZ has learned, the 19-year-old will walk Money May to the ring tomorrow before his next big boxing match in Vegas.

Bieber has been ingratiating himself with Mayweather since last year, when the singer appeared out of nowhere at Mayweather's match against Miguel Cotto ... walking the fighter to the ring, and celebrating with him afterwards (along with Lil Wayne).

The courtship continued ... and this summer, Mayweather flew Bieber out to his 13-year-old daughter's birthday party.

Now, Bieber's back for more shameless money-flaunting publicity whoring ... agreeing to accompany Mayweather to the ring Saturday at the MGM Grand, where May's set to fight Saúl "Canelo" Alvarez.

And he deserves to get his ass kicked. (We may or may not be talking about Alvarez.)