Simalungun Regency is a regency in North Sumatra Province of Indonesia. Its seat was formerly at Pematangsiantar, but this city was under Law No.15 of 10 March 1986 was separated from the Regency and made into an independent city (kota), although it remains geographically surrounded by the regency, whose new administrative seat is at Raya, while the regency's two most populous districts are Bandar (the centre of the Bandar urban area) and Siantar (essentially the eastern suburbs of Pematangsiantar city). The regency now covers an area of 4,372.5 square kilometres, and at the 2010 census it had a population of 817,720; at the 2020 Census this had risen to 990,246, but as of mid 2025 this was estimated to have declined to 955,620 (comprising 476,355 males and 479,265 females).
The regent of Simalungun is Radiapoh Hasiholan Sinaga, SH, who was elected as Regent of Simalungun on Pilkada (election of regional head) 2020, and took office on 21 April 2021.
The regency in 2010 was divided administratively into thirty-one districts (kecamatan), but in 2017 a 32nd district - Dolog Masagal - was created from parts of Raya District and Dolok Pardamean District. These are tabulated below with their areas and their populations at the 2010 Census and 2020 Census, together with the official estimates as of mid 2025. For ease of location these are grouped into three geographical sectors, which have no administrative significance. The table also includes the locations of the district administrative centres, the number of administrative villages in each district (totaling 386 rural desa and 27 urban kelurahan), and its post code.
Notes: (a) including one kelurahan. (b) not to be confused with Tigaraja in Girsang Sipangan Bolon District; in fact the administration of Pematang Silimahuta District would appear to be in Silimahuta Barat desa. (c) comprises 5 kelurahan (Baringin Raya, Dalig Raya, Merek Raya, Pematang Raya and Sondi Raya) and 12 desa. (d) The 2010 population of the new Dolog Masagal District is included in the figures for Raya District and Dolok Pardomean District, from which it was cut out in 2017. (e) includes 2 kelurahan - Aman Sari and Serbelawan. (f) eastern suburbs of Pematangsiantar City. (g) includes 2 kelurahan - Kerasaan I and Pematang Bandar. (h) includes 2 kelurahan - Perdagangan I and Perdagangan III. (j) comprises 3 kelurahan (Girsang, Parapat and Tigaraja) and 3 desa.
The urban area of Bandar comprises the four kecamatan (districts) of Pematang Bandar, Bandar Huluan, Bandar and Bandar Masilam, with a combined area of 387.4km2 and a population in mid 2025 of 171,819.
Six southern districts border directly on the north coast of Lake Toba; from west to east, these are Pematang Silimakuta, Silimakuta, Haranggaol Horison, Dolok Pardamean, Pematang Sidamanik, and Girsang Sipangan Bolon.
Local people mention it as Bukit Kapur Tinggi Raja, but some people have also called it "Kawah Putih" (White Crater), the same name as Kawah Putih in West Java. The crater is located in Dolok Tinggi Raja Village, 90 kilometres from Medan or 3 to 4 hours drive by motorcycle on the route Medan-Lubuk Pakam-Galang-Bangun Purba-Dolok Tinggi Raja.
