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Bulungan Regency, formerly known as Bulongan, is a regency of North Kalimantan Province in Indonesia. It covers an area of 13,181.92 km2 and had a population of 112,663 at the 2010 Census and 151,844 at the 2020 Census; the official estimate as at mid 2025 was 173,688. The administrative centre is at Tanjung Selor.

The modern iteration of Bulongan Regency (the original spelling) was established on 26 June 1959, replacing the earlier Special Region of Bulongan which was governed by the Sultanate of Bulungan (the last sultan: Maulana Muhammad Djalaluddin or Datuk Tiras). It was then one of the founder regencies of East Kalimantan during its establishment in 1956. It is unknown when exactly this regency changed its spelling into the current one.

Bulungan was originally co-extensive with what subsequently became the province of North Kalimantan. On 8 October 1997 the town of Tarakan was cut out of the regency to become an independent city. Two years later, on 4 October 1999, the remaining territory of the regency was split to create two additional regencies - Malinau Regency and Nunukan Regency. On 17 July 2007, three more districts (kecamatan) was split from Bulungan Regency to form a new regency, Tana Tidung Regency.

Bulangan Regency had originally been part of the East Kalimantan province, but in 2012 it - together with the areas carved out of it in 1997, 1999 and 2007 - became part of the newly created North Kalimantan province. Tanjung Selor in Bulungan Regency was designated as the province's capital.

Kabupaten Bulungan memiliki luas wilayah 13.181,92 km². Letak astronomisnya berada di antara 116°04’41” sampai dengan 117°57’56” Bujur Timur dan 2°09’19” sampai dengan 3°34’48” Lintang Utara.

Kabupaten Bulungan memiliki 10 kecamatan. Tiga Kecamatan terluas di Kabupaten Bulungan, yaitu: Kecamatan Peso seluas 3.142 km² atau 23,84% dari luas kabupaten, Kecamatan Sekatak seluas 1.993 km² atau 15,13% dari luas kabupaten, dan Kecamatan Tanjung Palas seluas 1.755 km² atau 13,32% dari luas kabupaten.

Wilayah Kabupaten Bulungan berbatasan dengan pelbagai kabupaten/kota, yakni:

Seperti wilayah lainnya di Pulau Kalimantan, Kabupaten Bulungan mengalami iklim hutan hujan tropis (Af) dengan suhu udara yang konstan antara 23°–33 °C dan curah hujan yang cenderung tinggi sepanjang tahun antara 2.800 hingga 3.400 mm per tahun. Berdasarkan pola curah hujan yang dimiliki, wilayah kabupaten ini termasuk dalam tipe pola hujan ekuatorial yang hanya memiliki satu musim, yakni musim penghujan.