Firefighters were called at about 7.30am (local time) after someone was heard banging on the wall and pleading for help.
It is believed the man was stuck inside a crawlspace for two days.
Rescuers cut through the theatre's walls and found him naked, local media said. He is being treated at a local hospital.
In a statement on Facebook, Syracuse fire department said it was not clear how he'd gained access to the crawlspace behind the wall.