Pematangsiantar, and also known as the City of Pematangsiantar, is an independent city in North Sumatra Province of Indonesia, surrounded by, but not part of, the Simalungun Regency, making Pematangsiantar an enclave within Simalungun Regency. Pematangsiantar formerly had the status of a second-level district and was the administrative centre of the surrounding Regency, but in 1986 it was elevated to Kota (City) and separated from the Regency.

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.