Justin Bieber Boxing with Muscle-Bound Trainer At NYC Gym

Justin Bieber got in the ring over the weekend to fight ANOTHER big guy -- and it wasn't terrible.

One month after throwing down with a dude twice his size in Cleveland, Bieber rolled into the gym at the Chelsea Piers Sports Center to work on his David vs. Goliath game.

Sources at the gym tell us ... Bieber arrived to the gym with the other dude, who they kept calling "Doc" -- and they sparred for almost 3 hours.

We're told Bieber was super nice and didn't pull any diva antics -- he was like any other gymgoer.

As for the fighting ... Bieber's been training with Floyd Mayweather ... who told us the pop star "ain't no bitch."