"This show would never have been the same without Paula and I can't believe I am saying this, [but] I have missed her a lot, and I am thrilled she's on the show," Cowell, 51, told Us Weekly in a statement Sunday afternoon, hours after his new series began its first official day of production in Los Angeles.
Abdul -- who exited Idol in 2009 after eight seasons -- tells Us she can't wait to start critiquing wannabe singers again.
"I am excited beyond words to be a part of The X Factor," the 48-year-old Abdul said in a statement. "I couldn't be happier to bring my love for cultivating talent and performance to such a wildly anticipated show.
"I'm looking forward to being back with the FOX/FremantleMedia family and know that it will be an extraordinary journey -- we're going to have a blast. I'm also delighted and grateful to be sitting next to Simon again...but you might want to check back with me in a week or two!"
Abdul's 11th hour appointment rounds out The X Factor's full celebrity roster, which includes fellow judges L.A. Reid and Cheryl Cole and cohosts Nicole Scherzinger and Steve Jones.