The two musicians were popped by the boxer to act as hype men for the bout that took place at the MGM Grand Arena in Las Vegas.
As the motely crew strutted down to the ring, a shirtless Weezy rapped his hit "A Milli" into a wireless mic while Biebs kept it, err, cool with black sunglasses and black clothes and that priceless affected Justin Bieber pout.
And it seems the musical posse helped Mayweather: he scored a majority-decision victory over Alvarez.
He also cashed in $41.5 million for the win. There's a reason the boxer's nickname is "Money": something all three dudes have very much in common.
For his part, Bieber was all about pimping the fight via social media. First he tweeted:
And then he Instagrammed a snap of him yucking it up (actually Biebs is just standing there pouting) with the champ before the game. The caption reads:
"Me and @floydmayweather right now before the fight."
But then, like any good bromantic partner, Mayweather returned the favor by also posting a pre-fight pic of him and the "Beauty and the Beat" singer hugging it out.
Meanwhile, Lil Wayne was spotted looking right at home watching the rest of the fight in the crowd sans shirt. Natch.