The 25-year-old victim was bleeding seriously and lapsing into shock when he was driven to a hospital in a pickup truck by some other surfers.
Deputies said the attack occurred around noon near the Eureka Municipal Airport. A witness told the Eureka Times-Standard they heard the victim calling for help and discovered a 14-inch wound on him.
The victim's name was not immediately released.
The attack was believed to be the work of a great white shark and came a week after a surfer was killed by an apparent great white while surfing north of Los Angeles near Lompoc, the Los Angeles Times said.