The Cool For the Summer singer will star in the comedy musical, alongside Avril Lavigne and Ashley Tisdale, and she has also been given the job of executive producing the film's soundtrack.
Chandelier singer Sia has written two tracks for the movie, including a song titled Magical, which will be sung by Lovato's character.
"There are not many performers confident enough to be part of the first soundtrack that I am executive producing," Demi says. "I am thrilled to welcome my friend and hero Sia to the movie."
Sia will also voice the character of Half Oracle in the film.
Meanwhile, Fall Out Boy frontman Stump wrote and produced a song called Trophy Boy, which will be performed by the film's other three female stars, Avril Lavigne, Ashley Tisdale and Chinese pop star G.E.M.
The trio play classic fairytale princesses who find out Prince Charming, voiced by Lovato's real-life boyfriend Wilmer Valderrama, is secretly engaged to each of them.