Simon Cowell has a new enemy ... Justin Bieber -- 'cause the pop star has teamed up with rival "X Factor" judge L.A. Reid to become a celebrity mentor on the upcoming season of the show. 

We have learned ... Bieber agreed to be one of 4 celebrity mentors assigned to one of the 4 judges ... Simon, L.A., Britney Spears and Demi Lovato. 

Bieber agreed to help out L.A.'s squad ... and yesterday, Justin went to a home in the Hollywood Hills to dish out some quality advice to the contestants. 

It's unclear which celebrities have been tapped to help out the other judges -- but good luck doing better than Bieber.

From TMZ.com

Thumbnail via TMZ.com