San Quentin State Prison spokesman Lt Sam Robinson says Ramirez died on Friday.
He had recently been taken from death row to a hospital.
Ramirez was convicted of 13 murders that terrorised Southern California in 1984 and '85, and sentenced to death.
Satanic symbols were left at murder scenes and some victims were forced to "swear to Satan" by the killer, who entered homes through unlocked windows and doors.
Ramirez was captured and beaten in 1985 by residents of a Los Angeles neighbourhood while attempting a carjacking.
