Joe Jonas Surprising Kid Cancer Patients with DNCE-THON

Joe Jonas and his DNCE mates are about to bust into a dance marathon for kids fighting cancer ... and turn the night into a full concert.

We got a pic of Joe and the band backstage Friday evening at THON ... the annual no sleep, no sit, 46 hour dance marathon on the Penn State campus. The event raises millions for cancer research, and this year organizers planned the DNCE surprise.

We're told Joe and co. plan to coolly walk out while the kids are doing a lip-synch battle to "Cake By The Ocean" -- and then launch into a 45 minute set.

Great gig for DNCE and a great cause.

Surprise went off without a hitch! Check the moves on that Nittany Lion.