Samuel Little, the United States' most prolific serial killer according to the FBI, died in California aged 80 on Wednesday, prison officials said.
Little confessed to 93 murders -- mostly of women -- carried out between 1970 and 2005, but his coast-to-coast killing spree went undetected for decades.
The former boxer's victims were mostly drug addicts and prostitutes, and in many cases the women were never identified or their deaths were not investigated.
After Little began naming victims from behind bars, the Federal Bureau of Investigation last year confirmed Little's involvement in at least 50 of the deaths, and described all his other claims as "credible."
He died in hospital early Wednesday, with Los Angeles coroners yet to determine the cause of death, the California Department of Corrections said in a statement.
Little was serving three consecutive life-without-parole sentences, after being jailed in 2014.
Also known as Samuel McDowell, Little stood 6ft 3in (1.9m) tall and typically knocked out his victims with powerful punches before strangling them, leaving no obvious signs of homicide such as stab marks or bullet wounds.
That factor combined with the victims' backgrounds meant many of the deaths were attributed to drug overdoses or accidents, as well as natural causes.
Little grew up in Ohio, dropped out of high school and lived a "nomadic life," shoplifting or stealing to buy alcohol and drugs, the FBI said.
His criminal record began in 1956 with arrests for shoplifting, fraud, drugs and breaking and entering.
He was accused of murdering women in Mississippi and Florida in the early 1980s but was not convicted.
US media reports recently said he suffered heart disease and diabetes.