Semaun (1899–1971), also spelled Semaoen, was the first chairman of the Communist Party of Indonesia (PKI) and was a leader of the Semarang branch of the Sarekat Islam.

A year later, in 1915, he met Sneevliet and was invited to join the Indische Sociaal-Democratische Vereeniging, the social democratic organisation of the Dutch East Indies.

At the same time, he was elected vice-chairman of the Surabaya office of the Indies Social Democratic Association (ISDV), which was to become the Indonesian Communist Party or PKI.

In 1918 he also became a member of the leadership council at Sarekat Islam (SI), then the dominant nationalist political organization in the Dutch East Indies.

On 23 May 1920, the Communist Party of Indonesia (originally the Partai Komunis Hindia, changed to 'Indonesia' a few months later) was founded after the deportation of the Dutch founders of the ISDV.

Together with Alimin and Darsono, Semaoen realised Sneevliet's ideals to enlarge and strengthen the communist movement in the Dutch East Indies.

The PKI was originally part of Sarekat Islam, but differences in understanding led the two major forces in SI to split in October 1921.

He remained involved as a nationalist activist on a limited basis, speaking a few times to Perhimpunan Indonesia, a Netherlands-based organization of Indonesian students.

He wrote a novel, Hikayat Kadirun, which combined communist and Islamic ideals, and produced a number of pamphlets and newspaper articles.