'Islamic Association'[1] or 'Islamic Union';[2] SI) was an Indonesian socio-political organization founded at the beginning of the 20th century during the Dutch colonial era.

From there, SI rapidly evolved into a nationalist political organization that demanded self-governance against the Dutch colonial regime and gained wide popular support.

[3] SI was eventually embroiled in an internal conflict between the Islamic moderates and the radical communist members who urged firmer anti-colonialist and anti-capitalist actions.

Under the charismatic leadership of Tjokroaminoto, who was hailed as a messianic figure or Ratu Adil, the organization rapidly expanded the network throughout Java and later spread to the outer islands.

[5][6] The rapid increase of SI membership led to the expansion of the supporter base to a wide array of social classes.

[6][12] While the initial supporter base was a petty bourgeoisie of religiously devout aliran (societal stream) of santri, the expansion had led to the inclusion of peasantry of abangan whose Islamic faith was mixed with mysticism and pre-Islamic animist beliefs, as well as members of priyayi nobility class who had secular outlooks.

The leadership of the organization gradually moved from small-scale merchants of the SDI era to the intelligentsia of priyayi origin with Dutch-language education.

[14] Tjokroaminoto declared the rejection of anti-government activity during the 1913 congress, during which he insisted on "loyalty" and "satisfaction" toward the Dutch government and denied the allegation that the organization served as a political party.

The radicalization was a result of the heightening Marxist-socialist influence in the city which led to the establishment of the Indies Social Democratic Association (ISDV) in 1914.

ISDV, founded by a Comintern agent Henk Sneevliet, was composed almost entirely of Dutch members, and they sought to spread their thoughts among the native Indonesian population.

[5] Tjokroaminoto finally answered their calls and demanded the Dutch colonial administration to form a parliament based on the popular election and alleviate labor issues, and refused to cooperate with the Volksraad unless the government addressed these grievances.

There was also a shooting incident in Garut, West Java in June 1919 which led to the arrest of SI leaders, including Sosrokardono and Tjokroaminoto who were imprisoned in 1921 for 11 months.

Salim and Soerjopranoto, a labor activist who belonged to the SI union and nicknamed raja mogok (the strike king), competed with Semaun for the leadership of PPKB.

The split of communist members, in addition to the mounting pressure from the colonial government, had led to the severe decline of SI as an organization.

In 1937, members such as Salim and Mohammad Roem were expelled from the party due to their non-confrontational stance against the Dutch and replaced by the more radical leadership of Abikusno Tjokrosujoso, Tjokroaminoto's brother.

It ceased to exist as an independent entity in 1973 after the forced merger into the United Development Party (PPP) at the hands of the Suharto regime.