Abdul announced her exit Tuesday, on the heels of the departures of "X Factor" judge Nicole Scherzinger and host Steve Jones.
That leaves Cowell and Antonio "L.A." Reid on the judging panel. There was no immediate word from producers on who might fill the open seats.
Abdul said in a statement that she's learned that "business decisions often times override personal considerations" and that she and Cowell remain friends.
Despite respectable ratings, "X Factor" has failed to achieve popularity similar to "American Idol," which Cowell left to import "X Factor" from the U.K. to the U.S.