New York State Park Police say the man jumped from the top deck to a seating area about 25 feet below during Wednesday night's concert. He was then airlifted to Nassau University Medical Center. He was not identified.
A spokeswoman said Thursday the man was in critical, but stable, condition.
One spectator below reported a minor injury. Police didn't say whether the jumper will face charges.
At least two concertgoers said the injury took place during an intermission in the popular jam-band's performance.
The show continued after the intermission without incident.