New York State Park Police say the young man pushed his way through the crowd, jumped off the balcony, and fell 25 feet onto chairs below.
He struck another concertgoer when he landed, That person sustained a minor injury. The jumper was airlifted to Nassau University Medical Center.
He was not identified.
A spokeswoman said Thursday the man was in critical, but stable, condition.
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.