The former Destiny's Child singer and the Mexican star will be joining Simon Cowell and Demi Lovato on the reality show, replacing Britney Spears and LA Reid.
Show boss SiCo said: "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."
And K-Row, who worked as a judge on the UK version of the show for one series to mixed reviews, said: "It feels great to be able to take this journey here at home in the States."
Rubio, who previously appeared as a coach on the Mexican version of The Voice, fired a shot across SiCo's bows.
She said: "I love The X Factor! 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. Empieza la fiesta, amigo!" ("The party's getting started, my friend!")
Reports that Cheryl Cole was being targeted for the show have come to nothing.
Watch MTV News on the hour every hour on MTV - Sky Channel 126 and Virgin Channel 134