The boxer was playing The $50k Charity Challenge celebrity basketball game at UCLA on Tuesday -- when he got posted up by the smokin' hot "Wild 'N Out" model.

Turns out, Elena has SERIOUS game ... and when Floyd when up for a shot, she packed him up good.

Floyd wanted a foul call -- he didn't get it.

Mayweather played pretty well for the most part -- aside from being CROSSED UP by R&B singer Tank.

By the way, props to all the people who played in the game and raised money for charity.

As for Elena, she's got a ton of modeling credits to her name -- but when we tried to find out if played college basketball, we couldn't seem to find any evidence confirming it.

If not ... some big school REALLY missed out.