Justin Bieber's Pool Time Fun with Mystery Blonde

Justin Bieber has nipples too, Greg ... and he was damn proud to show 'em off to a chick, poolside.

The Biebs was chilling outside near NYC Thursday -- with a mystery female playmate -- and having some good clean fun. No idea who the woman is, but she got an eyeful of Justin's tattoos and other body parts.

Bieber was totally relaxed for what looked to be a full day off. He'll be jetting off to England this weekend for Ariana Grande's One Love benefit concert.