The chatshow host will join fellow panellists Simon Cowell, Randy Jackson and Kara DioGuardi as the fourth judge on the ninth season of the Fox reality show.
"I've watched since the beginning, and I've always been a huge fan," she said.
"So getting this job is a dream come true, and think of all the money I'll save from not having to text in my vote."
Mike Darnell, president of Alternative Entertainment for Fox, added: "She is truly one of America's funniest people and a fantastic performer who understands what it's like to stand up in front of audiences and entertain them every day.
"We feel that her vast entertainment experience, combined with her quick wit and passion for music, will add a fresh new energy to the show."
DeGeneres will join the panel once the audition stages for the upcoming season are completed.