Khmer Rouge 'brother number two' Nuon Chea has died aged 93, said a spokesman for the Cambodia tribunal where he was convicted of genocide and crimes against humanity.
"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.
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.
He 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.