The singer will front hard-rock outfit The Fallen, featuring drummer Rocky Gray and guitarists Ben Moody and John LeCompt, as well as bassist Marty O'Brien.
Speaking to USA Today, she explained that the collaboration surfaced because of her friendship with Moody's flatmate.
"This all happened by accident," she said. "I met him once or twice [at the house] before the band thing."
The Fallen are scheduled to formally introduce themselves at a news conference in Los Angeles on Monday, which will also mark their first live performance.
Debut single 'Bury Me Alive' will be available for free download from their website following the event.
Irish-born Smithson finished in sixth place on the seventh season of the Fox reality show.