Large-scale killings and civil unrest primarily targeting members of the Communist Party (PKI) were carried out in Indonesia from 1965 to 1966. Other affected groups included alleged communist sympathisers, Gerwani women, trade unionists, ethnic Javanese Abangan, ethnic Chinese, atheists, so-called "unbelievers", and alleged leftists in general. According to the most widely published estimates at least 500,000 to 1.2 million people were killed, with some estimates going as high as two to three million. The atrocities, sometimes described as a genocide or politicide, were instigated by the Indonesian Army under Suharto. Research and declassified documents demonstrate the Indonesian authorities received support from foreign countries such as the United States and the United Kingdom.

The Communist Party of Indonesia was a communist party in the Dutch East Indies and later Indonesia. It was the largest non-ruling communist party in the world before its violent disbandment in 1965. The party had two million members in the 1955 elections, with 16 percent of the national vote and almost 30 percent of the vote in East Java. During most of the period immediately following the Indonesian Independence until the eradication of the PKI in 1965, it was a legal party operating openly in the country.