Sources close to JB tell us he recently bought himself a new pink sapphire grill ... which ran him somewhere in the ballpark of $15k.

The mouthpiece comes courtesy of Gold Teeth God -- a jeweler based out of L.A. -- who we're told Justin flew out to Miami last week to do an impression of his chompers.

The grill is a combo of pink sapphire and rose gold, with 6 teeth covered on top and another 6 on the bottom. It comes with 6 carats of sapphire ... and took about 5 days to make.

We're told Justin has bought jewelry from GTG before, so going to him for the grill was a no-brainer. He seems to dig it, too.

All we can say ... love your grill, Biebs.