The pair have long been rumoured for places on the upcoming 12th season's lineup since the departure of Jennifer Lopez and Steven Tyler.
Fox confirmed that the duo will join Mariah Carey and the returning Randy Jackson on the panel.
"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," said Mike Darnell, president of alternative entertainment, 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.
"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."
Executive producer Simon Fuller added: "With the exciting addition of Mariah Carey, Keith Urban and Nicki Minaj, alongside Randy Jackson, American Idol's star power has never been so great.
"For this we have to thank the ongoing commitment and belief of Fox and the dedication and focus of the Idol production team. We have all been together since the very beginning and we are focused on making Idol Season 12 the best ever."
Auditions for the 12th season begin later today (September 16), in New York City. Further audition cities will include Los Angeles, Chicago, Charlotte, San Antonio, Baton Rouge and Oklahoma City.
Minaj recently said that she was unsure whether she would join American Idol as a judge.
Urban quit The Voice Australia last week before signing up for the latest season of Idol.