Justin Bieber -- Tapped for 'X Factor'

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.

TMZ has 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.