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Kotabaru Regency is one of the eleven regencies in the Indonesian province of South Kalimantan. It consists of two parts; the smaller (2,361.61 km2) but more populated insular part comprises Laut Island ("Sea Island"), the largest island off the coast of Kalimantan (Indonesian Borneo), together with the smaller Sebuku Island off Laut Island's east coast and other even smaller islands nearby; the larger (6,993.82 km2) but less populated part consists of the 12 districts on the mainland of Kalimantan. The regency as a whole has an area of 9,354.93 km2, and had a population of 290,142 at the 2010 Census and 325,622 at the 2020 Census. The official estimate as at mid 2025 was 336,785 (comprising 173,678 males and 163,107 females), of whom 180,973 were in the insular part and 156,712 in the mainland part of the regency. The regency seat is located at the large town of Kotabaru at the northern tip of Laut Island.

It has the second largest GRDP in the province after the city of Banjarmasin, mainly due to its coal industry. It is also the largest regency by land area in the province.

Suku bangsa yang mendiami daerah ini antara lain: Suku Banjar Suku Bugis Suku Makassar Suku Mandar Suku Bajau Suku Dayak Bukit Suku Dayak Samihim Suku Jawa Suku Tionghoa-Indonesia

Kultur masyarakat di sini cukup beragam, sebagai dampak pembauran suku-suku di sini.

Lagu daerah dari kabupaten Kotabaru adalah: Paris Barantai, Kotabaru Gunungnya Bamega

Upacara adat di Kabupaten Kotabaru antara lain: Pawanangan, upacara suku Dayak Dusun di Sungai Durian, Kotabaru Papande Wanua/Papande Sasi, upacara suku Mandar di pulau Kerayaan Leut, upacara suku Bajau di Pulau Laut Utara, Kotabaru

The regency got its name from Kotabaru town, which is the administrative center of the regency. The town was previously a village that grew because of coal mining around 1873 and 1881. The coal mines were mostly owned by nobles of small kingdoms in the islands such as Pagatan and Kusan. The village grows into a town and called Kotta Baroe, which literally means "new town". Throughout the 20th century until today, the region became commonly known as Kotabaru.

In the region of what is now the regency, there were several petty kingdoms and sultanates which were closely tied to the Sultanate of Banjar. It is estimated that these kingdoms came into existence around after 1786, following the foundation of the Kusan and Pagatan Kingdoms on the island of Laut by Prince Amir from the Kingdom of Kayutangi (modern Martapura) Both kingdoms later on became subject to Kayutangi, and were obliged to pay tributes. In 1840, Kusan and Pagatan entered a personal union after the fourth king of Kusan, Jaya Sumitra, gave the kingdom to Arung Abdul Karim who was the king of Pagatan, thereby assuming control over the entire island of Laut. In 1881, son of Jaya Sumitra, Prince Husin Kusuma took the title of fourth king of Laut Island. Husin Kusuma soon died in 1900 when taking a hajj and was succeeded by Prince Aminullah who would become the last king of Laut.

In 1905, the Dutch East Indies government declared the end of the small kingdoms' existence on the island, following the defeat of the Banjar Sultanate after the Banjarmasin War. The region was then directly ruled by the colonial government instead of through local kings and officials. In 1942, the region was occupied by the Empire of Japan during World War II.

In the aftermath of the war and the Indonesian National Revolution, the returning Dutch proposed a puppet state of Southeast Borneo Federation in the region. This was opposed and conflicts erupted in the region, known as Kalimantan Physical Revolution. On 1 September 1945, Committee for Assisting Republic Indonesian (BPRI, Badan Pembantu Republik Indonesia) was formed in the town of Pagatan, and spread its branch across Laut Island. On 6 December 1945, students and Republican militia passed a motion declaring the region to be part of the Indonesian Republic. On 7 February 1946, a battle broke out between Republican militia and Dutch soldiers who tried to land on the beach of Laut island around the village of Kampung Baru.

Following the return of Dutch authority, several demonstrations were held in town of Pagatan and Kotabaru and Indonesian flag was hoisted in a wet market in Pagatan in October 1949. Southeast Borneo Federation were eventually dissolved by its own parliament and joined Indonesian Republic following transfer of sovereignty.

The regency on its own came into existence in 1953, when it was composed of administrative districts in Laut Island, Southern Tanah Bumbu, Northern Tanah Bumbu, and Pasir Residency. However, Pasir was cut off from the regency in 1959 after the division of Kalimantan into several provinces in 1956, and became a regency of East Kalimantan. Following the fall of Suharto and the rapid decentralization that followed, there was a demand from mainland districts (Tanah Bumbu) to secede from the regency and form their own; this demand has been caused by dissatisfaction on the mainland with the regency government on Laut Island, as the island is more developed than the mainland Kalimantan parts. The southern part of the Tanah Bumbu region (the southern 40.7% of the mainland area) was split off from the rest of Kotabaru Regency on 25 February 2003 and made into a separate regency. The northern part of the Tanah Bumbu region (the northern 59.3% of the mainland area) also demanded separation; however, by 2026 this additional split has not materialized because of the Indonesian Government moratorium on the created of new regencies.

Kotabaru has a tropical savanna climate (Aw) with heavy rainfall year-round.