The 35-year-old singer has been a permanent member of the judging panel since the show began in 2012. She won the first season of the show with mentee James Arthur.
Judges Gary Barlow and Louis Walsh also announced they were leaving the singing competition. However, Walsh is seemingly having second thoughts about his decision. Meanwhile, former judge Cheryl Cole is widely expected to return to the show.
Cowell, 54, who left the British series to establish the show across the Atlantic, comes back as his viewership expectations weren't met with the American franchise of the series The X Factor USA.
"[I'm] absolutely thrilled to be returning," Cowell reportedly said abut his comeback.
Showrunners are hoping that Cowell's return will boost the ratings of the British singing competition.