Justin Bieber Parties in Atlanta -- Hey Diddy, Check Out My Pecs

Justin Bieber was the belle of the ball Wednesday night as he revealed his boyish body -- to the amusement of a dozen big-time rappers at Diddy's Atlanta blowout.

Bieber's new GF, Chantel Jeffries, was in tow at Vanquish nightclub as he went around the room, mugging with Diddy, T.I., Jermaine Dupri, Wale, Rick Ross, Lil Scrappy and Trinidad James.

Bieber was a minority of one as he lost his shirt in the crowd but not his bling. And, even though there were lots of bottles poppin' ... Bieber stuck to bottled water the entire night.