The show's first-ever, four-judge panel will join host Ryan Seacrest in New York, where auditions take place Sunday and Monday, then travel to Los Angeles, Chicago, Charlotte, San Antonio, Baton Rouge and Oklahoma City, Fox said in a statement Sunday.
The 12th season begins in January.
Minaj is a multi-platinum singer, songwriter and rapper. Urban, married to actress Nicole Kidman, is a four-time Grammy award-winner.
"With an unparalleled star like Mariah, fan-favorite Randy, chart-toppers like Nicki and Keith and our incomparable host Ryan, we've put together one of the most exciting judging panels around," Mike Darnell, Fox Broadcasting Co.'s president of alternative entertainment, said in a statement.
Minaj, 29, posted news about the New York auditions on her Twitter page Sunday: "New York auditions today & tomorrow! See ya there barbz!!!!!!!!!"
Urban, 44, called it an "honor" to be part of the "Idol" family, People magazine reported.
"I'm looking forward to working with everyone and to seeing the same passion that I have for music in all of the participants," he said.