Justin Bieber -- 'F*** Pacquiao!!!'

Justin Bieber was amped up at Rehab in Vegas before the fight ... singing to Big Sean -- which is surprising -- and making his feelings about Manny Pacquiao clear.

Justin partied with the crowd before walking out with Floyd Mayweather, and at 33 seconds you can see him mouth the words, "f*** Pacquiao" ... he then flips the bird.

It's also interesting he's felling Big Sean, whose breaking with Ariana Grande did not go well. You'll recall he just gave her a squeeze from behind a few weeks back, which set Sean off. We're told Sean doesn't have hard feelings for Biebs ... he thinks Ariana just used him.