Borneo is the world's third-largest island, covering 748,168 km² in the centre of Maritime Southeast Asia, and since decolonisation it has been divided among three sovereign states. Indonesia holds about 73% of it (the five Kalimantan provinces), Malaysia about 26% (the states of Sarawak and Sabah), and Brunei the small remainder on the northwest coast. The equator runs almost through its middle. Its highest point is Mount Kinabalu in Sabah at 4,095 m, its longest river is the Kapuas at 1,143 km, and its 2020 censuses counted 23,053,723 people, most of them living along the coast and the lower reaches of the big rivers. The island is also where Indonesia is building Nusantara, its planned new capital.
Three Flags on One Island: Kalimantan, Sarawak, Sabah and Brunei
The political map of Borneo is a product of the nineteenth century. The Sultanate of Brunei once held influence over much of the coast. In 1841 it granted Sarawak to the Englishman James Brooke, whose family ruled it as the "White Rajahs" until 1946. The north passed, through concessions from the sultanates of Brunei and Sulu, to the British North Borneo Chartered Company, the forerunner of Sabah. The rest of the island, the vast south and east, fell under Dutch control. When the colonial era ended, the Dutch part passed to Indonesia, while Sarawak and Sabah joined the new Federation of Malaysia in September 1963. Brunei stayed out of Malaysia and became fully independent in 1984.
The result is a sharply unequal split. Kalimantan, as Indonesians call both their portion and the whole island, covers about 534,698 km² across the provinces of West, Central, South, East and North Kalimantan. Sarawak is Malaysia's largest state at about 124,450 km², nearly the size of Peninsular Malaysia, and Sabah covers about 73,904 km². Brunei, at 5,765 km², is tiny by comparison and is itself cut in two: the Sarawak district of Limbang separates its main western part from the mountainous Temburong District. It is the only sovereign country whose territory lies wholly on Borneo; Indonesia and Malaysia both extend far beyond it.
Population follows a different pattern from area. Sabah counted 3,418,785 people in the 2020 census and Sarawak about 2.45 million, so the Malaysian quarter of the island holds well under a third of its people. Brunei had about 466,000 people in 2025. The rest, more than 16 million, live in Kalimantan, concentrated in river-mouth cities such as Pontianak on the Kapuas, Banjarmasin near the Barito, Samarinda on the Mahakam and the oil port of Balikpapan. Among the many Indigenous peoples of the interior, often grouped under the broad label Dayak, are the Iban and Bidayuh of Sarawak and the Kayan, Kenyah and Ngaju of Kalimantan, many of whom historically lived in communal longhouses along the rivers.
Mount Kinabalu and the Crocker Range
The northern tip of Borneo holds the island's most dramatic relief. Mount Kinabalu rises to 4,095 m at Low's Peak in Ranau district, Sabah, making it the highest mountain in Malaysia and in Maritime Southeast Asia and the 20th most prominent peak in the world. That figure dates from a satellite re-survey in 1997, which trimmed 6 m off the long-published height of 4,101 m. Geologically the mountain is young: it is a granodiorite pluton that pushed up through folded sedimentary and ultrabasic rocks and cooled only about 10 million years ago, and it is still gaining roughly 5 mm of height a year. During the Pleistocene its summit carried ice, which explains the bare, sculpted granite plateau and jagged pinnacles climbers see today.
Kinabalu Park, created in 1964 and covering 754 km² around the mountain, became Malaysia's first UNESCO World Heritage Site in December 2000. Its altitude range, from lowland forest to alpine shrubland, packs 5,000 to 6,000 plant species and 326 bird species onto a single massif. In 2010 the park received 611,624 visitors, including 47,613 climbers, all of whom must be accompanied by accredited guides.
Kinabalu sits at the northern end of the Crocker Range, which runs southwest down Sabah and divides the state's West Coast from its Interior. The range averages about 1,800 m, is made of uplifted and folded sandstones and shales, and is named after William Maunder Crocker, a nineteenth-century British administrator of North Borneo. Crocker Range National Park was gazetted in 1984 and UNESCO recognised the range as a Biosphere Reserve in 2014. On its eastern flank lies the Tambunan Valley at about 800 m, a basin of terraced rice paddies, and its slopes feed rivers flowing both west to the South China Sea, such as the Papar and Padas, and east into the interior.
The Interior Spine: Müller Mountains and the Heart of Borneo
South of Sabah, Borneo's high ground continues as a broad, forested chain that carries the Malaysia–Indonesia border across the island. Away from Kinabalu the summits are modest: Mount Trusmadi in Sabah, Malaysia's second-highest peak, reaches 2,642 m, and the sandstone Mount Murud, at 2,424 m, is the highest point of Sarawak. The Müller Range in the centre of the island is a hydrological hub, the source of both the Kapuas, flowing west, and the Barito, flowing south, while the Mahakam rises nearby at Cemaru and runs east.
Much of this interior is limestone and cave country. In Sarawak's Gunung Mulu National Park, a UNESCO World Heritage Site, the Clearwater Cave holds one of the longest underground rivers in the world, and Deer Cave shelters more than three million bats, whose guano has piled up to more than 100 m deep. In East Kalimantan, the Sangkulirang-Mangkalihat karst holds thousands of smaller caves.
In February 2007 the governments of Brunei, Indonesia and Malaysia signed the Heart of Borneo declaration in Bali, a World Wide Fund for Nature initiative to conserve about 220,000 km² of this forested interior. It links existing protected areas including Kinabalu, Crocker Range and Gunung Mulu in Malaysia, Kayan Mentarang and Betung Kerihun in Indonesia, and Ulu Temburong in Brunei. Between 2007 and 2010 alone, 123 species new to science were recorded in the region.
Kapuas, Barito and Mahakam: Borneo's River Highways
Borneo's rivers are long, slow and heavily used, because roads in the interior are few. The Kapuas in West Kalimantan is the longest river on the island and in Indonesia, at 1,143 km, and one of the longest island rivers anywhere. It rises in the Müller Range, runs through mountainous terrain for only its first 165 km or so, then crosses a marshy plain where it falls just 50 m over the 902 km from Putussibau to its delta west of Pontianak. Its basin, about 99,000 km², covers more than two-thirds of West Kalimantan. About 350 km from the source the river feeds the Kapuas Lakes, a set of seasonal floodplain lakes including Sentarum and Luar, which swell when monthly rain tops about 300 mm and act as a natural reservoir for the river.
The Barito in South Kalimantan is the second longest at about 1,090 km. The Mahakam, the third, flows about 980 km from Long Apari in the highlands to the Makassar Strait, draining roughly 77,100 km² of East Kalimantan. Its middle course crosses a lowland of peat and some thirty shallow lakes, and its lower reach carries the provincial capital, Samarinda, about 48 km from the mouth. The Mahakam is home to a small freshwater population of Irrawaddy dolphins, roughly 70 animals in a 420 km stretch, where most recorded deaths come from entanglement in gillnets.
On the Malaysian side the rivers are shorter but just as important. The Rajang, at 565 km, is Malaysia's longest river and carries boat traffic deep into central Sarawak; the Bakun Dam stands on its tributary the Balui. In Sabah the 560 km Kinabatangan, Malaysia's second longest, winds through a floodplain of oxbow lakes and is the only river where the Borneo river shark has been recorded.
Rainforest Loss, 2002–2019
Borneo's lowland rainforest is estimated at around 140 million years old and is dominated by dipterocarp trees; the island has about 15,000 flowering plant species, 3,000 tree species, 221 land mammals and 420 resident bird species. Lowland rainforest alone was mapped by WWF at about 427,500 km². That cover has shrunk fast. According to Global Forest Watch data, Indonesian Borneo lost 10.7 million hectares of tree cover between 2002 and 2019, including 4 million hectares of primary forest, while Malaysian Borneo lost 4.4 million hectares, 1.9 million of it primary. Logging for tropical timber came first, followed by conversion to oil palm plantations that now press against the last lowland primary forest.
Fire has compounded the losses. Since 1997, land-clearing fires during dry El Niño years have burned into drained peat swamp forest along the coasts, producing haze that reached as far as southern Thailand, Cambodia, Vietnam and the Philippines in 2015. The toll on wildlife is measurable: a 2018 study estimated that Bornean orangutan numbers fell by about 148,500 between 1999 and 2015. The forests that remain are still a stronghold for the Borneo elephant and the Bornean clouded leopard, and the island's apes are covered in more depth in our guide to orangutans and the geography of their habitat.
Nusantara: Indonesia's Capital Project in East Kalimantan
In August 2019 President Joko Widodo announced that Indonesia's capital would move from sinking, overcrowded Jakarta on Java to Borneo, and in September 2021 his government sent a capital-relocation bill to parliament. The new city, Nusantara, is being built in the East Kalimantan regencies of Penajam North Paser and Kutai Kartanegara, beside the port city of Balikpapan and just south of the Mahakam delta. Construction began in 2022, and the project is planned in five phases running to 2045, the centenary of Indonesian independence, at an estimated cost of Rp 523 trillion (about US$35 billion).
The move has slipped. Nusantara was due to be inaugurated as the capital on 17 August 2024, and Indonesia did celebrate Independence Day there for the first time that day, but the move itself did not take place; the guest list was cut from 8,000 to 1,300 because facilities were unfinished. By the end of Widodo's term the State Palace and ministry offices were complete. Under President Prabowo Subianto the project has continued, with Rp 48.8 trillion allocated for its second phase. For the island it means a new city carved out of plantation and forest land on the east coast, unlike the river towns that have defined Borneo's settlement for centuries.
Whether you start from Kinabalu's granite or the peat swamps of the Kapuas, the island's story keeps returning to its forest; our guide to the Borneo rainforest follows that thread, and the ranking of the world's largest islands shows how Borneo compares with Greenland and New Guinea.