Cowell, 54, is leaving The X Factor USA after three seasons, which will conclude the domestic run of the competition series.
"Apart from being lucky enough to find some amazing talent on [The X Factor USA and American Idol], I have always had an incredible welcome from the American public (most of the time!)," said Cowell. "Last year, for a number of reasons, I had to make a decision to return to the U.K. version of The X Factor in 2014. So for now, I'm back to the U.K. and I want to thank FOX for being an incredible partner and I also want to thank everybody who has supported my shows. America, I'll see you soon!"
"Unfortunately, there is no X Factor USA without Simon Cowell, but we understand and support his decision to focus on the international formats and on the next phase of his personal life," said Kevin Reilly, Chairman of Entertainment, Fox Broadcasting Company. "We wish him the very best, and it's our sincere hope that we work together again soon."
Alex & Sierra were the last winners of the show in 2013. The duo reached No. 1 on the iTunes singles chart twice. The show also won a 2013 People's Choice award for Favorite TV Competition Show.