The shark, believed to be a 20-foot great white, bit 27-year-old Eric Tarantino while he was surfing at Marina State Beach in Monterey County.
Tarantino was bitten on the neck and forearm. His surf board showed a 19-inch bite mark.
A fellow surfer who was in the water with him says he heard his friend yelling "shark" repeatedly.
"He just said call 911," said witness Brandon McKibbin. "At that point I looked over and he was only a couple feet away from me and you could see his arm was bleeding pretty badly."
Witnesses say the two men paddled back to shore where other surfers came to Tarantino's aid before he was taken to the hospital.
California state park officials say it's likely that the shark has left, but they are still encouraging people to stay out of the water.