Comrade Duch, a former senior figure of the Khmer Rouge convicted of crimes against humanity in Cambodia, has died.
He was serving a life sentence after being sentenced by a UN-backed court.
Kaing Guek Eav, known as Comrade Duch, ran the notorious Tuol Sleng prison where thousands of people were tortured and murdered in the late 1970s.
As many as two million people are believed to have died under the Khmer Rouge, a Maoist regime that controlled Cambodia from 1975 to 1979.
Duch was the first senior Khmer Rouge leader convicted of crimes against humanity by a UN-backed tribunal in 2010 and sentenced in 2012.
He passed away on Wednesday, aged 77, a spokesperson for the tribunal said, without giving details of the cause. He had been ill for many years.
"Duch died this morning at 00:52am, on 2 September at Khmer Soviet Friendship Hospital. Details of what he died of, I can't tell," Khmer Rouge tribunal spokesman Neth Pheaktra said.
He ran the S-21 prison, also known as Tuol Sleng. It is thought that at least 15,000 men, women and children deemed enemies of the regime have passed through the gates of that facility.
Most of them were tortured, forced to confess to alleged crimes against the Khmer Rouge and then put to death at the so-called killing fields just outside the capital Phnom Penh.
During his trial, Duch admitted he was in charge of S-21 and apologised for his part in the horrors committed there.
He later claimed he had only been following orders, but his appeal on those grounds was rejected by the tribunal.
During the many years of the Khmer Rouge tribunal he later testified against two deputies of the regime's leader Pol Pot.