Minaj and Urban, who quit the Australian version of 'The Voice' earlier this month, will join fellow newcomer Mariah Carey and long-serving panellist Randy Jackson on the US talent show, reported Rolling Stone magazine.
"So excited to be joining the team on American Idol," Urban posted.
'American Idol' host Ryan Seacrest also took to the social networking site to confirm the four judges will be on the panel for the forthcoming 12th season of the show.
"It's official! Your Season 12 Idol judges are Mariah Carey, Randy Jackson, Nicki Minaj and Keith Urban. Meet the judges (sic)," he wrote.
Randy, who has been a judge on the programme since it began in 2002, signed a new deal to remain on the show after talks with Spanish singer Enrique Iglesias broke down.
But 'X Factor' judge Simon Cowell, who was a panellist on 'American Idol' from 2002 to 2010, says keeping Randy on was the right decision.
"Just heard Randy is back on Idol. Right decision," Cowell posted.