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 and Siantar. 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, of whom 497,314 were males and 492,932 were females; the official estimate as at mid 2023 was 1,035,920 - comprising 521,262 males and 514,658 females.

The principal group of inhabitants of the Regency (and of Pematangsiantar city, an enclave within the regency) are the Simalungun people, a sub-group of the Batak people. Their language (Simalungun language) is an Austronesian language.

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.

• North: Serdang Bedagai Regency and Deli Serdang Regency.
• East: Batubara Regency and Asahan Regency.
• South: Toba Regency.
• West: Karo Regency and Lake Toba.

The regency in 2010 was divided administratively into thirty-one districts (kecamatan), but in 2017 a 32nd district - Dolog Masagal - was created. These are tabulated below with their areas and their populations at the 2010 Census and 2020 Census, together with the official estimates as at mid 2023. 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.

Kode Wilayah	Name of District (kecamatan)	Area in km	Pop'n Census 2010	Pop'n Census 2020	Pop'n Estimate mid 2023	Admin centre	No. of villages	Post code
12.08.25	Silimakuta	74.16	13,611	17,479	18,590	Saribu Dolok	7	21169
12.08.31	Pematang Silimakuta	79.68	10,334	13,390	14,278	Tiga Raja	10	21167
12.08.14	Purba	172.71	21,830	27,536	29,134	Tiga Runggu	14	21166
12.08.15	Haranggaol Horison	40.97	4,994	7,041	7,694	Haranggaol	5	21177
12.08.20	Dolok Pardamean	67.90	16,008	15,136	15,732	Dolok Pardamean	11	21163
12.08.09	Sidamanik	80.88	26,953	30,853	31,702	Sari Matondang	15	21171
12.08.10	Pematang Sidamanik	137.80	16,283	19,716	20,614	Sait Buttu Saribu	10	21188
12.08.16	Girsang Sipangan Bolon	129.89	14,325	17,941	18,942	Parapat	6	21174
12.08.11	Tanah Jawa	174.33	45,568	54,900	56,957	Pematang Tanah Jawa	20	21181
12.08.12	Hatonduhan	336.26	21,140	26,632	28,167	Buntu Bayu	9	21178
12.08.13	Dolok Panribuan	148.62	17,947	22,018	23,109	Tiga Dolok	15	21173
12.08.06	Jorlong Hataran	93.70	15,316	19,154	20,215	Tiga Balata	8	21172
12.08.04	Panei	77.96	21,425	28,326	30,382	Panei	17	21161
12.08.05	Panombean Panei	73.74	19,193	24,993	26,689	Pematang Panombean	11	21165
12.08.29	Raya	261.56	30,876	30,150	31,591	Pematan Raya	17	21160
12.08.32	Dolog Masagal	105.77		13,406	14,694	Bah Bolon	10	21164
12.08.26	Dolok Silau	302.66	13,716	17,696	18,846	Saran Padang	14	21168
12.08.27	Silau Kahean	228.74	17,000	19,477	20,040	Negeri Dolok	16	21157
12.08.07	Raya Kahean	204.89	17,398	21,583	22,725	Sindar Raya	14	21156
12.08.28	Tapian Dolok	119.89	38,034	45,032	46,778	Sinaksak	11	21154
12.08.17	Dolok Batu Nanggar	106.91	39,364	45,659	47,147	Serbelawan	16	21155
12.08.01	Siantar	73.99	62,916	73,536	76,101	Dolok Marlawan	17	21151
12.08.02	Gunung Malela	96.74	32,676	41,189	43,571	Dolok Malela	16	21175
12.08.03	Gunung Maligas	51.39	26,173	33,734	35,916	Silau Bayu	9	21176
12.08.18	Huta Bayu Raja	191.43	29,135	35,426	37,085	Hutabayu	16	21182
12.08.19	Jawa Maraja Bah Jambi	38.97	19,951	22,892	23,564	Jawa Maraja	8	21153
12.08.21	Pematang Bandar	88.16	31,324	38,688	40,684	Pematang Bandar	13	21186
12.08.22	Bandar Huluan	107.33	25,738	29,484	30,335	Naga Jaya I	10	21184
12.08.23	Bandar	100.69	63,584	79,006	83,226	Perdagangan	16	21162 - 21184
12.08.24	Bandar Masilam	91.22	24,316	30,015	31,558	Bandar Masilam	10	21185
12.08.08	Bosar Maligas	285.43	38,970	43,355	44,229	Bosar Maligas	17	21183
12.08.30	Ujung Padang	228.49	40,522	44,803	45,624	Ujung Padang	20	21187
	Totals	4,372.50	817,720	990,246	1,035,920	Raya	413

Note: (a) including one kelurahan. (b) comprises 3 kelurahan (Girsang, Parapat and Tiga Raja) and 3 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 figure for Raya District, from which it was cut out in 2017.
(e) includes 2 kelurahan - Aman Sari and Serbelawan. (f) includes 2 kelurahan - Kerasaan I and Pematang Bandar.
(g) includes 2 kelurahan - Perdagangan I and Perdagangan III.

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.

Dayang Bandir and Sandean Raja is a folk story from Simalungun.

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.

Parapat is a small town located on the shore of Lake Toba. This place is known for tourism, and ferry services to Samosir Island are also available.

2°54′00″N 99°00′00″E / 2.9000°N 99.0000°E / 2.9000; 99.0000