The pair will join pop star Mariah Carey and Randy Jackson, the only remaining original "Idol" judge. The announcement came just hours before initial auditions for the next season in New York.
"I am thrilled about this year's judges' panel!" said Mike Darnell, president of alternative entertainment at the Fox Broadcasting Company.
"Nicki's an unbelievably captivating international phenomenon who has made an indelible mark on rap and pop. And Keith is another great addition to IDOL – he's one of the biggest stars in country music and I know that our fans and contestants will fall in love with him."
He said the now-complete team was "one of the most exciting judging panels around."
Pop diva Jennifer Lopez and Aerosmith's Steven Tyler announced in July their departure from the show, which is struggling against rival "The Voice" on NBC.
"American Idol" has aired for 11 seasons, but was thrown into disarray by the departure of judges Simon Cowell, Ellen DeGeneres and Kara DioGuardi two years ago.
But Marc Graboff, president of the CORE Media Group that owns "Idol" producer 19 Entertainment, insisted that season 12 would be "terrific."
"IDOL has truly never looked better," he added. "We couldn't be more pleased about this new team of judges."
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