Justin Bieber Playing Basketball -- Shirts or Skins ... Still A Baller! [VIDEO]

Justin Bieber shocked a gym full of people yesterday -- not with his basketball skills, but by keeping his shirt ON while he was balling.

Going topless has kinda become JB's trademark, yet he hit the hardwood at Equinox in West L.A. for a pickup game with his buds ... and actually wore a black tank (and headband) while playing.

Justin didn't totally disappoint though. Check out our hot, sweaty footage of him leaving the gym.