The singer-and-actress and the Aerosmith frontman will join original judge Randy Jackson on the 10th season of the talent searching show, which will air early next year.
Auditions will be held in Los Angeles later today (22.09.10) before heading to New Jersey next week.
Jennifer and Steven join the panel following the departures of Simon Cowell, Ellen DeGeneres and Kara DioGuardi.
It had been reported Jennifer had agreed a $12 million deal to be a judge on 'American Idol', after bosses allegedly turned down her demands for a $15 million deal.
A source revealed: "I think it was just positioning. In the end, she didn't turn up her nose up at $12 million."
Lopez's manager Benny Medina arranged a meeting between the 'Jenny From the Block' singer and producers in June and she accepted the judging role in July.