You flex those muscles, Justin Bieber.

The 19-year-old singer -- who, at this point, may be just as well-known for his shirtless selfies as he is for his music -- gave fans an even bigger thrill on Wednesday when he Instagrammed an ab-baring picture with his personal trainer, showing off his toned physique.

Though the Biebs has pretty much always rocked a six-pack, this new shot proves just how ripped he has gotten -- all thanks to that awesome trainer, who isn't too bad-looking himself, FYI.

And Bieber makes sure to give credit where credit is due, purposely pointing out the fitness guru --  a move that just so happens to perfectly flaunt his own bulging biceps.

Get those tickets to the gun show ready, ladies.