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.

Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guinea. Indonesia is the world's largest archipelagic state and the 14th-largest country by area, at 1,904,569 square kilometres. With over 279 million people, Indonesia is the world's fourth-most-populous country and the most populous Muslim-majority country. Java, the world's most populous island, is home to more than half of the country's population.