At least 14,000 people were imprisoned at what is now the Tuol Sleng Genocide Museum.
Fewer than a dozen are believed to have lived to tell the tale.
Chum Meth, 49, is one of eight known survivors of S-21 and the only woman.
In an exclusive interview with Al Jazeera, she told Hamish Macdonald that for almost 30 years she had been harbouring the secret that she had once been a prisoner at S-21.
She spent two weeks inside the prison and her grainy mug-shot now appears in the prison museum.
But until recently, the mother of three who works as a cook at a local school had not even told her husband her secret.
"My memory from when I was inside the prison is of hearing the sounds of pain, I could hear them torturing the bodies.
"When I think about it I am nearly sick, I feel like I have a fever. It is very painful for me to talk about."
Her testimony may prove crucial in the trial of the senior Khmer Rouge figures.