Adam's death led to a change in police procedures over missing children
A serial killer who died in prison in 1996 was guilty of the murder and decapitation of a six-year-old boy 27 years ago, say Florida police.
The naming of Ottis Toole as Adam Walsh's killer brings an end to one of America's best-known unsolved cases.
Police said the discovery was made after a thorough review of records.
The case radically affected police procedures over missing children and inspired the TV show "America's Most Wanted", presented by Adam's father.
Adam went missing from a shop in Hollwood in 1981. His head was recovered from a canal two weeks later but his body was never found.
Toole, a convicted killer and paedophile, had long been suspected of involvement in the murder and had admitted guilt at least twice, but later recanted.
At a news conference, Hollywood Police Chief Chad Wagner said that as Toole was dead, further legal action was not possible and the Walsh family were not seeking it.
"However, in the interest in justice, the Hollywood Police Department is announcing today that it is our determination and conclusion that Ottis Toole was the abductor and murderer of Adam Walsh," said Mr Wagner.
Mr Walsh said not knowing who killed Adam had been torture
John Walsh, Adam's father told the news conference: "For 27 years we've been asking, 'Who could take a six-year-old boy and murder him and decapitate him?'.
"We needed to know and today we know. The not knowing, it's been a torture, but that journey is over."
Mr Walsh had been critical of what was widely seen as a botched police investigation, but he praised the police for finally bringing an end to the case.
"This is not to look back and point fingers, but it is to let it rest," he said.
The murder of Adam, and Mr Walsh's subsequent campaigning, led to a raft of measures being introduced, including hotlines to report cases and images of missing children being printed on milk cartons.
Mr Walsh also presents the long-running true life crime programme America's Most Wanted, launched in 1988, which calls on the public to help identify fugitive suspects.
In 2006, US President George W Bush signed the Adam Walsh Child Protection and Safety Act, giving state authorities greater powers to track down and prosecute child sex offenders and to make it harder for abusers to reach children online.