The 29-year-old, who was axed from the US version show previously, will help fellow judge Gary Barlow mentor acts in later rounds, reported Sun Online.
"Welcome back Cheryl," Cowell tweeted.
Her entry to the show was kept under the wraps by Barlow, who orchestrated the plan of having Cole on the show.
"Only Gary's intimate inner circle know. He hasn't even discussed it with Simon or ITV bosses. It's been seriously top secret stuff.
"They're all under the impression it's going to be Robbie (Williams). No one even noticed the dates for the recording clashed with Robbie's new solo tour," a source said.
Cowell, 52, was kept in dark about Cole's return to the show following the singer's feud with him for axing her in America. The pair have since patched things up.
