Middleton became an overnight celebrity after she served as maid of honor at her sister Kate's wedding to Britain's Prince William in April.
"I haven't heard her sing, but if she walked in and said, 'I want to be a TV star,' I would sign her on the spot," The Sun newspaper quoted Cowell as saying. "She has the X Factor. To overshadow your sister when she's the one getting married? That was impressive."
Cowell has served as a judge on the talent competition shows "American Idol," "Britain's Got Talent" and the British series "The X Factor." He will soon be seen on the upcoming U.S. edition of "The X Factor."