The Tuol Sleng Genocide Museum (Khmer: សារមន្ទីរឧក្រិដ្ឋកម្មប្រល័យពូជសាសន៍ទួលស្លែង) is a museum in Phnom Penh, chronicling the Cambodian genocide. The site is a former high school which was used as the notorious Security Prison 21 (S-21) by the Khmer Rouge regime from its rise to power in 1975 to its fall in 1979. Tuol Sleng (Khmer: ទួលស្លែង) means "Hill of the Poisonous Trees" or "Strychnine Hill". Tuol Sleng was just one of at least 150 execution centers established by the Khmer Rouge. On the 26 July 2010, the Extraordinary Chambers in the Courts of Cambodia convicted the chief of Tuol Sleng Prison, Kaing kek Iew, (Alias Duch) for crimes against humanity and grave breaches of the 1949 Geneva Conventions and sentenced him to life imprisonment.

From 1975 to 1979, an estimated 17,000 people were imprisoned at Tuol Sleng (some estimates suggest a number as high as 20,000, although the real number is unknown). At any one time, the prison held between 1,000–1,500 prisoners. They were repeatedly tortured and coerced into naming family members and close associates, who were in turn arrested, tortured and killed. In the early months of S-21's existence, most of the victims were from the previous Lon Nol regime and included soldiers, government officials, as well as academics, doctors, teachers, students, factory workers, monks, engineers, etc. Later, the party leadership's paranoia turned on its own ranks and purges throughout the country saw thousands of party activists and their families brought to Tuol Sleng and murdered.[3] Those arrested included some of the highest ranking communist politicians such as Khoy Thoun, Vorn Vet and Hu Nim. Although the official reason for their arrest was "espionage", these men may have been viewed by Khmer Rouge leader Pol Pot as potential leaders of a coup against him. Prisoners' families were often brought en masse to be interrogated and later executed at the Choeung Ek extermination center.

In 1979, the prison was uncovered by the invading Vietnamese army. In 1980, the prison was reopened by the government of the People's Republic of Kampuchea as a historical museum memorializing the actions of the Khmer Rouge regime.