Justin Bieber -- Mayweather, Meet Selena

Looks like Justin Bieber is done living the bachelor life ... he actually brought his girlfriend Selena Gomez to Las Vegas for the Floyd Mayweather fight -- and she's gonna meet his boxing BFF.

Sources close to Bieber tell us the couple hit up the Hard Rock pool party Saturday and Selena was spotted with friends in a picture posted on Shots.com. Bieber also posted a pic poolside.

We're told Selena and Floyd have surprisingly never met -- Biebs usually rolls to Vegas with his boys -- but he plans to introduce the two after the fight.