The duo will join Mariah Carey and long-serving judge Randy Jackson on the panel of the US reality TV juggernaut.
Urban has previously been a coach on the Australian version of The Voice, so has a bit of previous form for talent shows.
An official picture of the four judges with host Ryan Seacrest has been released by Fox.
Mike Darnell, president of alternative entertainment at Fox, said: "We've got global icon Mariah Carey and Idol's heart and soul Randy Jackson, who will be joined by Nicki Minaj and Keith Urban.
"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.
"With an unparalleled star like Mariah, fan-favourite Randy, chart-toppers like Nicki and Keith and our incomparable host Ryan, we've put together one of the most exciting judging panels around."
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