The attack occurred on Saturday near San Onofre State Beach in northern San Diego County.
See also: Tourist loses leg in Hawaii shark attack
Thomas Williams, one of several witnesses who pulled the woman ashore, told the Orange County Register: "All of the back of her leg was kind of missing."
Williams said the woman was "not calm" but was conscious, and that a rubber surfboard leash was used as a tourniquet to stop the bleeding.
According to the Guardian, it's thought the attack was made by a great white shark or a seven-gill shark.
The Telegraph reports that the beach is adjacent to the Camp Pendleton Marine Base. Marine Sgt Asia Sorenson said the victim - a civilian - was airlifted to a hospital in unknown condition.
The paper added that last year, a woman was bitten by a shark while swimming off Corona del Mar, about 25 miles to the north of the Saturday attack.
Experts said at the time that the the bite marks on her chest, hip and shoulder suggested that the shark was at least 10ft in length.