The Extraordinary Chambers in the Courts of Cambodia is examining crimes committed by members of the Khmer Rouge regime that ruled the country during the 1970s.
The trials in Phnom Penh come more than 30 years after the fall of a government responsible for the deaths of hundreds of thousands of Cambodians.
The tribunal announced Thursday that Ieng Thirith, the former social affairs minister, likely suffers from Alzheimer's disease and was unfit to stand trial.
"In these circumstances, and as the co-prosecutors and defense have acknowledged, the chamber is obliged to order the accused's release from detention," the tribunal stated.
The court said Ieng Thirith, 80, allegedly bore direct responsibility for crimes against humanity, genocide and other grave breaches of the Geneva Conventions.
She's the fourth surviving member of top leadership of the regime to face the tribunal. She is the wife of former Khmer Rouge Foreign Minister leng Sary.
Kaing Guek Eav, also known as Duch, is serving a life sentence for crimes committed while he was chairman of the Phnom Penh detention and interrogation center known as S-21.