Gereja Kristen Protestan Simalungun (GKPS - Simalungun Protestant Christian Church) is a Lutheran and Reformed Protestant church formally founded to spread Christianity among the Simalungun people, a tribe living in Simalungun, North Sumatra, Indonesia. It has a baptized membership of 211,383.
August Theis led a group of Dutch missionaries to spread Christianity to the Simalungun area after he received a telegram from the local Rheinische Missionsgesellschaft (RMG - Netherlands-based sending institution) leader. On September 2, 1903 they arrived at Pematang Raya to begin their mission among the Simalungunese. September 2 has been celebrated each year by GKPS all over the World as an olob-olob (Simalungunese for joyous) day to thank God that the Bible has entered Simalungun.