"Singing with me ... Miley Cyrus! Let's hear it!" Idol said during his set which also included the 60-year-old performing past hits including "Dancing With Myself" and "White Wedding."
"I'm singing with Billy [expletive] Idol, let's do it!" Cyrus said as she emerged from the back shocking fans.
Cyrus, a noted fan of Idol's, paid tribute to the '80s icon by wearing sunglasses with his name written across it, a jacket featuring his face on the back and black leather pants that read "I [love] Billy Idol" on both legs.
The pop star has performed "Rebel Yell" before in 2012 during VH1's Divas Live concert.
The first night of the 2016 iHeartRadio music festival on Friday also featured performances from Drake, Sia, One Republic, U2 and more. On Saturday, Britney Spears, Arianna Grande and Sting took the stage.