The attack took place Friday morning at Surf Beach in Santa Barbara County.
Authorities are releasing few details, but a coroner's investigator has been called to the location.
The victim, a body surfer, reportedly "had his leg bitten off and lost too much blood."
The coroner has not confirmed this account.
Ann Blodzinski, a spokeswoman at the nearby Vandenberg Air Force Base, says officials have closed Surf, Wall and Minuteman beaches for 72 hours.
The victim was not associated with the base.
Surf Beach is located north of Santa Barbara and west of Lompoc on Vandenberg Air Force Base.