The 79-year-old is thought to suffer from Alzheimer's disease and dementia.
The three most senior surviving Khmer Rouge leaders are on trial for crimes against humanity.
They include Ieng Thirith's husband Ieng Sary, who was the regime's foreign minister.
Officials estimate that the Khmer Rouge killed up to 2.2 million people during its 1975-1979 rule.
Under supreme leader Pol Pot, the regime tried to build a Maoist peasant utopia, but descended into genocide as paranoid leaders sought to eliminate anyone who threatened them.
Ieng Thirith served as the regime's social affairs minister, and was arrested and charged with the other leaders.
Who were the Khmer Rouge?
- Maoist regime that ruled Cambodia from 1975-79
- Led by Saloth Sar, better known as Pol Pot
- Abolished religion, schools and currency in effort to create agrarian utopia
- Up to two million people thought to have died of starvation, overwork or by execution
- Defeated in Vietnamese invasion in 1979
- Pol Pot fled and remained free until 1997 - he died a year later
The judges added that she would be transferred to medical facility for treatment "which may help improve her mental health to such an extent that she becomes fit to stand trial".
The other leaders on trial are Pol Pot's deputy, Nuon Chea, and Khieu Samphan, the regime's former head of state.
All three deny the charges they face.
The UN-backed court's first case was the trial of Duch, the former Khmer Rouge prison chief who oversaw the torture and execution of thousands of inmates at Tuol Sleng prison.
He was convicted of crimes against humanity last year.