Dutch police investigating the death of schoolboy Nicky Verstappen in 1998 say their prime suspect has been arrested in northern Spain.
Jos Brech was identified as a suspect after the Netherlands' biggest ever DNA harvesting operation, which included his relatives.
It gave police a vital clue linking him to DNA on Nicky Verstappen's clothes.
Mr Brech is due to be extradited to his home country, where he faces charges of kidnapping, sexual assault and murder.
"Jos B, 55, a suspect in the death of Nicky Verstappen was arrested in Spain on Sunday afternoon. He was taken into custody and will be handed over to the Netherlands," said a statement from police in Limburg, the southern Dutch province where the boy disappeared.
The arrest follows a public appeal last week, during which detectives shared photographs of the suspect. A Dutchman living in Spain recognised the man in the photos and tipped off police.
Nicky, then aged 11, was at a summer camp in August 1998 when he vanished from his tent. His body was discovered in the forest the following day.
Jos Brech, who lived nearby, was reported missing earlier this year, following the largest DNA testing project ever in the Netherlands.
One of the only pieces of evidence in the case was DNA discovered on Nicky's clothing.
More than 14,000 men voluntarily provided samples to the investigation. Police hoped a relative of the person whose DNA was found would take part, thus narrowing the search.
Mr Brech did not participate. He had been spotted near the scene of the crime in 1998. He was later interviewed as a witness by police, but was thought to be an innocent passer-by.
He travelled to eastern France last year and was reported missing by his family in April. Police then examined his home and discovered he was a 100% match for the DNA found on Nicky's body.
A killing that shocked the Netherlands
By Anna Holligan, BBC News, The Hague
This murder mystery has horrified and captivated the Dutch for two decades.
Nicky Verstappen, the freckle-faced schoolboy, was adopted as a symbol of an unfathomable, faceless threat to our children.
Now the prime suspect in his murder is in custody in Spain, and a European arrest warrant issued last week is expected to facilitate his return to the country where the grotesque crime was committed.
The Netherlands is buzzing with a desire for justice, and in some cases revenge.
If Jos Brech denies the charges, the challenge for prosecutors will be to ensure emotion does not affect the suspect's right to a fair hearing in court.
Nicky's mother Berthie told Dutch TV on Sunday night said the family did not dare believe that the arrest would come so soon after the police appeal on 22 August. "We hope there'll be quick answers now to all the questions we have," she said.
Spanish police were tipped off by a Dutchman who said he had seen him in the village of Castelltercol, north of Barcelona, and spoken to him in July.
"He'd been living in a tent in the woods, close to a house that several people have visited for years. It's a sort of commune where I had lived for ages too," the man told the Telegraaf newspaper.
A leading defence lawyer said he was prepared to take on the case on Jos Brech's behalf as there were traces of DNA from other people on the body as well.