Justin Bieber is to appear on the US version of The X Factor.

The pop star has reportedly been tapped by the hit singing contest to appear as a celebrity mentor on the upcoming season of the show. His appearance reportedly annoyed head judge Simon Cowell, who wanted Justin to mentor alongside him at the judges' homes phase of the competition. Instead, the 18-year-old opted to team up with L.A. Reid.

"Bieber agreed to be one of 4 celebrity mentors assigned to one of the 4 judges, Simon, L.A., Britney Spears and Demi Lovato," TMZ reports. "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."

The Canadian singer-songwriter is the second most popular celebrity on Twitter. He has over 20 million followers on the social network site and chose to tweet his thanks to L.A. Reid at the end of the day of filming.

"Long day but great day. @LA_Reid just like old times! Thanks (sic)," Justin wrote.

It is not known which other celebrities are being tapped to star as mentors.