Appearing in the King Country Superior Court on Wednesday, Gary Leon Ridgway, changed his previous plea of not guilty and admitted murdering the women in a killing spree that spanned two decades, beginning 1982.
His confession effectively ends a long running mystery over the identity of the Green River Killer – a sobriquet earned after the lifeless bodies of the initial victims were fished out of the Green River south of Seattle.
Asked if he understood that he admitted guilt to 48 counts of aggravated murder, Ridgway replied: "Yes I do."
As a result, Ridgway is poised to enter the history books as the worst serial killer in US history.
His confession came two years after his arrest in 2001. He owned up to his horrific crimes only after prosecutors agreed not to press for the death penalty.
Ridgway had been charged with seven killings, but was suspected in 49 murders between 1982 and 1984 and perhaps dozens more since then, including the murders of two women in 1990 and 1998 that police recently solved with his help.
Ridgway was considered a prime suspect in the case for years before a breakthrough DNA test linked him to several victims.
Though his initial victims were fished out of the Green River, later bodies showed up at various spots in western Washington.
He mostly preyed on women, who were either drug addicts or prostitutes.
"I wanted to kill as many women as I thought were prostitutes as I possibly could," he said in the statement.
Most of them - strangled between 1982 and 1984 - were prostitutes who worked on "the strip" close to Sea-Tac airport, which serves Seattle and nearby Tacoma.
Ridgway now heads a list of notorious serial killers in the US, including Robert Lett Yates Jr, who admitted killing 15, and Ted Bundy, who confessed to 28 murders in the 1970s.