Former Khmer Rouge commander Chhouk Rin has lost a final appeal against his conviction in connection with the 1994 murder of three Western tourists.
A previous appeal court also found Chhouk Rin guilty
The five-judge panel said Chhouk Rin, who was not present in court, should be arrested and jailed immediately.
Chhouk Rin did not take part in the murders himself, the court decided.
But there was evidence he ordered his unit of Khmer Rouge fighters to join an attack on the train in which the three backpackers were taken, the court said.
"The... decision on the sentence of Chhouk Rin according to the anti-terrorism law is correct," said presiding judge Khim Pon.
"The Supreme Court decided to uphold the verdict," he added.
Mark Slater from Britain, Australian David Wilson and Jean-Michel Braquet from France were ambushed on a train travelling from Phnom Penh to the seaside resort of Sihanoukville.
The three men were killed when ransom negotiations with the government failed. Thirteen Cambodians also died.
At the time of the kidnapping, Chhouk Rin was a mid-ranking Khmer Rouge commander, although he had defected to the government by the time the foreigners' bodies were discovered.
Two Khmer Rouge commanders, senior to Chhouk Rin, are already serving life sentences over the deaths.
Chhouk Rin was originally sentenced in September 2002, but he was allowed to remain free while both his appeals were considered.