Sary, arrested in November 2007, was being tried before the Extraordinary Chambers in the Courts of Cambodia on charges of genocide and crimes against humanity committed during the Khmer Rouge regime in the late 1970s, the United Nations said in a release.
He had been hospitalized several times since his detention, the latest on March 4, the United Nations said.
"The death of the accused Ieng Sary has the effect of terminating all criminal and civil actions ... before the ECCC," the trial chamber said in its decision.
Sary was on trial along with his wife, Ieng Thirith, who served as social affairs minister during the Khmer Rouge regime's bloody leadership of the country. Former head of state Khieu Samphan is on trial as well.
In September, the court found Thirith unfit to stand trial for to medical reasons and granted her a provisional release. Psychiatrists diagnosed her with clinical dementia, which they said would hinder her participation in court hearings.
An estimated 2 million people died during the Khmer Rouge regime between 1975 and 1979.