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