The crime happened just before 1 a.m. Tuesday at the Garden of Eden store on Garden Grove Boulevard.
Police said two suspects, one of them armed with a gun, walked into the adult bookstore and demanded money from the clerk.
Things turned violent when one of the suspects hit the clerk on top of his head with the gun.
Police said both robbers ran off and are still on the loose.
Anyone with information was urged to call Garden Grove police at (714) 741-5704.