Rowland, 32, and Rubio, 41, are replacing LA Reid and Britney Spears from the third season of the singing talent hunt, reported Ace Showbiz.
"It's taken more than a decade, but I'm delighted to finally be on a panel with three girls (I think!). Paulina and Kelly both have great taste and massive experience in the music industry and together with Demi, this is going to be a fun panel. It just feels like the time to do something different," Cowell said in a statement.
"I am very excited to be reuniting with Simon Cowell and The X Factor family. It feels great to be able to take this journey here at home in the States!," she said.
Rubio, who sold over 20 million albums in a career spanning over three decades, said, "I can't wait to find the next big music star here in America. I'm so thrilled to be part of the show as a judge. Simon, be careful what you wish for, let's see if you are ready to handle me...