Cowell will be both executive producer and judge on "The X Factor."
"I'm thrilled that we have put a date on the launch of the U.S. version of 'The X Factor,' and delighted to be continuing to work with Fox," said Cowell in a statement. "We have a fantastic relationship, a great team and are all very excited about this." iReport: Will you watch "Idol" without Simon?
Speculation has raged for weeks as to Cowell's future. Though he's well paid for his "Idol" work -- an estimated $36 million per season -- he has expressed a desire to focus on "The X Factor."
Cowell said at the press session that it's taken some time to reach an agreement for his new duties, hence the buzz. (Indeed, he signed the last page of his "contract" on stage with Fox executives Peter Rice and Kevin Reilly.)
"[I] didn't think it was right to do two shows in America at the same time, so [I] decided to leave one and start another," he said.
He said he missed Paula Abdul, the former "Idol" judge who left the show in August, but wouldn't confirm or deny whether she would appear on "X Factor" in the U.S. Cowell and Abdul had a good-naturedly contentious relationship on "Idol," with the tough-minded Brit often acting as bad cop to the "Forever Your Girl" singer's good cop.
Ellen DeGeneres, who's taking over Abdul's judging slot, joked about Cowell's announcement on her talk show Monday -- getting in a dig at NBC's late-night troubles as well.