TEHRAN, (YJC) - "We can confirm that defendant Nuon Chea...passed away this evening on 4 August 2019 at Khmer Soviet Friendship hospital," said Neth Pheaktra, spokesman for the tribunal.
The cause of his death was not given but he had been in hospital since early last month.
The reign of terror led by "Brother Number 1" Pol Pot left some two million Cambodians dead from overwork, starvation and mass executions from 1975 to 1979.
But Nuon Chea, considered the Khmer Rouge's chief ideologue, was not arrested until 2007.
He and other senior members of the ultra-Maoist group were put on trial at the Extraordinary Chambers in the Courts of Cambodia.
The UN-backed court sentenced him to life in prison last year after he was found guilty of genocide against the ethnic Vietnamese and Cham Muslim minority group.
His lawyers had informed the court that Nuon Chea would appeal, but prosecutors are now expected to ask the Supreme Court chamber to terminate his case following his death, according to a court official.
Nuon Chea and the sole surviving defendant on trial, Khieu Samphan, were previously handed life sentences in 2014 over the forced evacuation of Phnom Penh in 1975, when Khmer Rouge troops drove the population of the capital into the countryside.
The revolutionaries who tried to recreate Buddhist-majority Cambodia into an agrarian Marxist utopia attempted to abolish class while targeting religious groups and the educated.
The hybrid court, which uses a mix of Cambodian and international law, was created with UN backing in 2006 to try senior Khmer Rouge leaders.
It has convicted only three people so far and cost more than $300 million.
Former Khmer Rouge foreign minister Ieng Sary and his wife died without facing justice, while Pol Pot passed away in 1998.
Prime Minister Hun Sen, a former Khmer Rouge cadre, has warned against future investigations, claiming it would plunge the country into chaos.
Tribunal watchers believe that last year's ruling will be the final verdict, raising questions about the court's legacy in a country where many are too young to remember the Khmer Rouge.
Late Sunday police stood guard outside the geriatric centre at the Khmer Soviet Friendship hospital in the Cambodian capital while reporters gathered outside.
Youk Chhang, head of the country's pre-eminent Khmer Rouge archive, the Documentation Centre of Cambodia, said Nuon Chea was "born innocent but he committed sin and so he died with sin.”
"The crimes he has committed will always be a lesson for us all in the future."