The Tuol Sleng Genocide Museum, or simply Tuol Sleng, is a museum chronicling the Cambodian genocide. Located in Phnom Penh, the site is a former secondary school which was used as Security Prison 21 by the Khmer Rouge regime from 1975 until its fall in 1979. From 1976 to 1979, an estimated 20,000 people were imprisoned at Tuol Sleng and it was one of between 150 and 196 torture and execution centers established by the Khmer Rouge and the secret police known as the Santebal. On 26 July 2010, the Extraordinary Chambers in the Courts of Cambodia convicted the prison's chief, Kang Kek Iew, for crimes against humanity and grave breaches of the 1949 Geneva Conventions. He died on 2 September 2020 while serving a life sentence.

The Cambodian genocide was the systematic persecution and killing of Cambodian citizens by the Khmer Rouge under the leadership of Prime Minister of Democratic Kampuchea, Pol Pot. It resulted in the deaths of 1.5 to 2 million people from 1975 to 1979, nearly 25% of Cambodia's population in 1975.