The 26-year-old former Jonas Brothers singer joined Lovato and sibling Nick Jonas on their Future Now tour to perform "Gotta Find You" and "This is Me" from the Disney Channel movie.
"Sharing the stage with @nickjonas and @ddlovato in DC last night!" Jonas captioned a photo of himself embracing Lovato onstage.
"Surprising Demi with Gotta Find You was something I will never forget! Thanks for having me guys! Amazing show you two put on. Proud to have such talented wonderful people in my life."
Jonas and brothers Kevin and Nick Jonas co-starred with Lovato in Camp Rock (2008) and its sequel, Camp Rock 2: The Final Jam (2010). The singer and Lovato dated the same year of The Final Jam's release.
"She's family. We've known Demi for so many years," he gushed in an interview with E! News on Monday. "We've performed with her, toured with her. She's just incredible."
"I don't think of her as an ex," the star added. "We're really good friends. We've gone through so much together and we're really close now."
Lovato and Nick Jonas kicked off their tour June 29 and will bring the venture to a close Sept. 17. Jonas and his band DNCE are presently touring with fellow Disney Channel star Selena Gomez.