"It's official! Cheryl is back. Be careful what you wish for," Cowell tweeted.
"Most importantly, she is a brilliant judge of talent," he said.
The 'Girls Aloud' star was a judge on the UK show through 2010 before briefly serving as a judge on the now-canceled US version.
Cowell recently announced his return to the show after a three-year absence. The last time Cowell was a judge on the UK version, the final episode drew 17.2 million viewers.