Here is a short history of the Indonesian Communist Party (Partai Komunis Indonesia, PKI) in Aceh province, northern Sumatra, beginning with a brief overview of the effect of Dutch colonization on Acehnese society. It then traces the PKI’s emergence in the province during the early 1920s and the PKI’s relationship with political Islam between the 1930s and 1950s, before outlining the complexities of the PKI’s involvement in Aceh’s political life during the early 1960s. It argues that there is no reason to believe that the PKI in Aceh was particularly small or deviated substantially from the PKI’s patterns of growth or membership elsewhere in Indonesia. It also proposes that the relationship between the PKI and political Islam in the province was more complex than is generally acknowledged. It was only after a military-led anti-Communist propaganda campaign during the time of the 1965–66 genocide (Thirtieth of September Movement) that the PKI came to be widely viewed as a mortal enemy of Islam by Aceh’s civilian population.