Samuel Little had been serving three consecutive life sentences for murdering three women in the late 1980s at the time of his death, the California Department of Corrections and Rehabilitation said in a statement.
Little had been suffering from diabetes, heart trouble and other ailments. The cause of death is being determined by the coroner.
In 2018, Little confessed to killing 93 people — mostly in Florida and Southern California — between 1970 and 2005.
So far, authorities have confirmed Little had been responsible for nearly 60 slayings, and continue to investigate his claims of the other murders.
Investigators say most of his victims were prostitutes, drug addicts and poor people, many of whom he strangled.
Little was busted in 2014 after DNA linked him to the murders of the three women. He was convicted of first degree murder and sentenced to life in state prison.