Justin Bieber has teamed up with old school R&B group Boyz II Men for the new single "Fa La La" ... off his new album Under The Mistletoe.

The black and white video shows a very grown-up looking Bieber dancing and singing alongside the foursome. The "Baby" singer has admitted to being a fan of Boyz II Men since he was a little kid. Wait ... isn't he still a kid?

Wanya, one of the four members of the band, recalls a funny story involving Bieber and their 90s hit song, "I'll Make Love To You." He tells MTV News:

    "[Justin] told us his mum didn't want him to sing I'll Make Love to You, so he'd sneak and go sing it somewhere."

Awkward! Despite the very adult lyrics, we have a feeling Biebs sounded phenomenal!