Justin Bieber will perform a new song he's recorded with Boyz II Men on "Dancing with the Stars" ... TooFab has learned exclusively.

We're told Justin and Boyz will perform "Fa La La" -- a cappella -- on the results show next Tuesday.

We snagged a video of Justin and the men from Boyz recording the song. "Fa La La" is a track on Bieber's holiday album "Under the Mistletoe."

Check it out above!