(Gereja Kristen Protestan Simalungun, GKPS)
This church, autonomous since 1963, is concentrated mainly in North Sumatra, among the approximately 300,000 people who speak Simalungun, a Batak dialect, and has congregations in Java where the church has followed its people. The GKPS traces its beginnings back to the work of the Rhenish Mission in the Batak area in the 19th century. About 70 percent of its members are farmers; others are engaged in various occupations in urban centres in North Sumatra and in the nation's capital Jakarta. A translation of the entire Bible in the Simalungun language by Simalungun scholars, begun in 1957, was completed in 1969. Among the Simalungun people are some 5,000 Roman Catholics and several thousand Muslims. The church continues to work among the latter.

The church organization combines congregational and synodal features. An ephorus and a general secretary, elected once in five years, head the church. Church headquarters in Pematang Siantar are conveniently near the HKBP Theological Seminary where the church's pastors are trained; other students attend Abdi Sabda Theological Seminary in Medan, Jakarta Theological Seminary, and the seminary in Jogyakarta. The church maintains many elementary and secondary schools. It has a development service, contributing drinking water supplies to the village people, motivating rice growing, poultry raising, cattle breeding. Its medical clinic, "Bethesda" in Saribu Dolok and in Pematangraya, helps to relate health services to the community needs.

The GKPS has close ties with the Evangelical Lutheran Church in America and the Lutheran Church in Australia and some districts of UEM members in Germany.