Adam, who was 6 in 1981, was abducted from a Hollywood, Fla., shopping mall and his head was discovered two weeks later. His other remains were never recovered.
Police on Tuesday named a convicted serial killer named Ottis Toole, who died in prison in 1996, as the child's suspected murderer.
"It is our determination and conclusion that Ottis Toole was the abductor and murderer of Adam Walsh," said Hollywood Police Chief Chad Wagner. "If [he] were alive today, he would be arrested."
Wagner did not elaborate on any new evidence that the police have uncovered.
Toole, a drifter who was serving time for an unrelated murder, twice confessed to killing Adam, only to recant in both instances. Toole, it was discovered, would sometimes confess to dozens of murders he did not, in fact, commit.
In a 1983 confession about Adam, Toole reportedly cited a machete as the weapon used to decapitate the boy.
Transformed John WalshWalsh's son's death galvanized the former hotel marketing executive into one of the country's leading activists on behalf of missing children.
"Who could take a 6-year-old and murder and decapitate him? Who?" Walsh, 62, said at a news conference in Florida. "We needed to know. We needed to know. And today we know. The not knowing has been a torture, but that journey's over."
Walsh instituted the Adam Walsh Child Resource Center and co-founded the National Center for Missing and Exploited Children. His America's Most Wanted TV show launched in 1988.
In its 24 years, the center has assisted authorities in more than 148,160 missing child cases, and helped in the recovery of more than 132,300 children.