Lovato, 21, is making a guest appearance on the Fox musical series, reported Us magazine.
"She's playing my love interest, so I'm very, very excited about that. We get to sing a song together. It's gonna be good. I'm excited to work with her.
"We're bringing her to the dark side. Well, she comes in in a Beatles episode, so (we're singing) a Beatles song. I'm excited," Rivera said.
Lovato also tweeted her excitement about joining 'Glee' and said, "Way too excited to be starting Glee this week! I can't believe I'm going back to acting but the best part about it is that I get to act-sing with some of my really great friends..."