Rihanna Guest Judge on 'X-Factor'

Simon Cowell is pulling out all the stops to make sure the U.S. version of "The X Factor" is a hit -- we've learned he's snagged Rihanna as a guest judge.

Judges on the show bring on guests to help them decide which contestants to mentor -- and Antonio "L.A." Reid, who signed Rihanna back in 2005, will bring her on board to help him decide.

According to our sources ... RiRi loved doing the U.K. finale last year, and the feeling from Simon was obviously mutual.