Andre Berto -- Bets $100k on Mayweather ... To Beat Pacquiao

When in Vegas ...

Another rich celebrity is getting into the gambling spirit -- with boxing superstar Andre Berto dropping $100k to bet on his boy Floyd Mayweather ... TMZ Sports has learned.

We're told Berto placed the bet last night at the MGM Grand.

The odds are so strongly in Floyd's favor, that even if Berto wins ... he only stands to make about $45,000.

Still, not a bad night.

As we previously reported, there are a bunch of huge, rich stars dropping big cash on the fight ... including 50 Cent, who says he's betting $1.6 on Floyd.