The show boss - who is currently judging on The X Factor USA - appeared via video link during rehearsals for this week's show.
He informed the remaining eight contestants that they will all perform on next year's X Factor arena tour.
He said: "Hi guys, sorry I couldn't be here tonight. I'm in America but I'm here to make an important announcement. All of you will be taking part in the X Factor tour this year.
"The hard work starts now. You'e got to be focused, you've got to be amazing and you've got to sell millions of records. Good luck, I'm watching."
Cowell has not judged on the UK series since 2010, when he mentored One Direction to third place in the competition.
He is reportedly considering a return to the show next year to replace the departing Gary Barlow.
The X Factor continues tomorrow night (November 10) at 8pm on ITV.