Ramirez, 53, had been taken from San Quentin's death row to Marin General Hospital, where authorities said he died of liver failure.
He had been housed on death row for decades and was awaiting execution, even though it has been years since anyone has been put to death in California.
Ramirez was convicted in 1989 of 13 Los Angeles-area murders, as well as charges of rape, sodomy, oral copulation, burglary and attempted murder, and was sentenced to death. The murders in 1984 and 1985 terrorized Southern California, with satanic symbols left at bloody murder scenes and some victims being forced to "swear to Satan" by the killer, who entered homes through unlocked windows and doors.
His marathon trial was a horror show in which jurors heard about one victim's eyes being gouged out and another's head being nearly severed. Courtroom observers wept when survivors of some of the attacks testified.
Ramirez was captured and beaten in 1985 by angry residents of an East Los Angeles neighborhood who recognized him as he tried to carjack a woman in her driveway.
At his first court appearance, Ramirez raised a hand with a pentagram drawn on it and yelled, "Hail, Satan."
The trial of Ramirez took a year, but the entire case which was bogged down in pretrial motions and appeals lasted four years, one of the longest criminal cases in U.S. history.
Because of the notoriety of the case, more than 1600 prospective jurors were called.
After his conviction, Ramirez flashed a two-fingered "devil sign" to photographers and muttered a single word: "Evil."
In 2006, the California Supreme Court upheld Ramirez's convictions and death sentence. The U.S. Supreme Court refused in 2007 to review the convictions and sentence.
Two years later, San Francisco police said DNA linked Ramirez to the April 10, 1984, killing of 9-year-old Mei Leung. She was killed in the basement of a residential hotel in San Francisco's Tenderloin neighborhood where she lived with her family.
Ramirez had been staying at nearby hotels.
Ramirez previously was tied to killings in Northern California. He was charged in the shooting deaths of Peter Pan, 66, and his wife, Barbara, in 1985 just before his arrest in Los Angeles, but he was never tried in that case.
The Associated Press contributed to this report.