Demi Lovato & Joe Jonas Share ‘Camp Rock’ Reunion at Jonas Brothers Concert

The Jonas Brothers left Disney fans stunned at their New Jersey concert ... bringing out a surprise guest that had everyone feeling nostalgic.

It all went down Sunday night at MetLife Stadium ... where the JoBros were reflecting over their 2008 smash hit Disney movie, "Camp Rock." Joe Jonas -- the film's lead -- broke out into song, singing "Gotta Find You," sending fans into a frenzy.

But then ... he brought out Demi Lovato onstage -- who played his onscreen love interest -- and the whole crowd went wild.

The duo then belted out some of the most popular songs from the movie, like "Wouldn't Change a Thing" and "This is Me."

It was a long time coming for this reunion ... after "This is Me," Demi and Joe told the crowd they hadn't performed the song live in roughly a decade.

What makes this all the more interesting ... Demi and Joe had briefly dated in 2010 -- but continued a solid friendship following their split.

In addition to Demi, the brother band also brought out Jesse McCartney to perform his hit song "Beautiful Soul," among others -- and their former bodyguard Big Rob for "Burnin' Up."