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;^[2] at the 2020 Census this had risen to 990,246, of whom 497,314 were males and 492,932 were females;^[3] the official estimate as at mid 2023 was 1,035,920 - comprising 521,262 males and 514,658 females.^[1]

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^[2] and 2020 Census,^[3] together with the official estimates as at mid 2023.^[1] 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.

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.