Indonesia has three land neighbours: Malaysia on the island of Borneo, Papua New Guinea on New Guinea and Timor-Leste on Timor. Measured the way the international reference articles measure them, those borders run 1,881 km, 824 km and 253 km, a total of about 2,958 km; Indonesia's own boundary figures put the total slightly higher, at 3,108.3 km. At sea the list is much longer. Indonesia shares maritime boundaries with Australia, India, Malaysia, Palau, Papua New Guinea, the Philippines, Singapore, Thailand, Timor-Leste and Vietnam, ten countries in all, and its exclusive economic zone near the Natuna Islands is also overlapped by China's nine-dash line claim. For a country of between 13,000 and 17,000 islands, the world's largest archipelagic state, the sea boundaries matter at least as much as the three short stretches of land.
Borneo: 1,881 km with Malaysia under the 1891 London Convention
Indonesia's longest land border crosses Borneo, the island it shares with Malaysia and Brunei. Brunei sits on the north coast entirely inside Malaysian territory, so it never touches Indonesia. The Indonesia–Malaysia line runs for 1,881 km from Tanjung Datu, the north-western corner of Borneo, through the forested interior highlands to the Celebes Sea in the east. South of it lie the Indonesian provinces of West Kalimantan, East Kalimantan and North Kalimantan; north of it are the Malaysian states of Sarawak and Sabah.
The line is colonial. After the British adventurer James Brooke established himself in Sarawak in 1841, the Dutch in Batavia issued a decree in February 1846 dividing the island along its watersheds. That logic carried into the Border Convention signed in London between the United Kingdom and the Netherlands on 20 June 1891, which remains the principal document behind today's frontier. Further agreements in 1915 and 1928 fine-tuned it, especially around Jagoi and Stass in western Borneo, and Indonesia and Malaysia inherited the result as successor states.
At its eastern end the convention fixed the boundary at 4°10′ North and carried it straight across Sebatik, an island of about 452 km² off the Sabah coast. The northern part of Sebatik is Malaysian, the southern part Indonesian.
Official road crossings are few and all of them link West Kalimantan with Sarawak. The main one is Entikong–Tebedu, on the road between Pontianak and Kuching; the others are Aruk–Biawak, Nanga Badau–Lubok Antu and Jagoi Babang–Serikin. The frontier is porous, and many undocumented Indonesian workers cross it into Malaysia. Borneo's importance to Jakarta is rising: Nusantara, the planned new national capital, has been under construction since 2022 in East Kalimantan, near the port of Balikpapan. For the rest of the island's story, see our guide to the geography of Borneo.
New Guinea: the 141st Meridian Line with Papua New Guinea
New Guinea is split almost exactly in half, and the 824 km border that does it consists of two straight north–south lines joined by a short stretch of river. It starts on the north coast immediately west of the Papua New Guinean village of Wutung and runs due south along the 141st meridian east, across the Bewani Mountains and the Central Highlands. When it reaches the Fly River it follows the river in a C-shaped curve, then resumes as a second straight line at 141°01′10″ East down to the mouth of the Bensbach River on the Torres Strait. On the Indonesian side are the provinces of Papua, Highland Papua and South Papua; on the other side are Sandaun and Western provinces.
The straight line is a product of three empires. The Netherlands claimed the north-west coast of New Guinea as far as the 140th meridian in 1828. Germany took the north-east as Kaiser-Wilhelmsland in 1884 and Britain the south-east, and in 1895 Britain and the Netherlands signed the treaty that put their common boundary where it is now. British New Guinea passed to Australia in 1902. When Indonesia won recognised independence in 1949, the Dutch kept western New Guinea; it was transferred to Indonesia only in 1963. Australia, still administering Papua New Guinea, signed the boundary agreement with Indonesia in February 1973, two years before Papua New Guinea became independent in 1975.
The frontier has rarely been quiet. The West Papuan insurgency that began after 1963 has pushed refugees eastward; more than 10,000 Papuans crossed in 1984 alone. Indonesia and Papua New Guinea signed a friendship treaty in 1986, renewed in 1990, committing both to settle border issues peacefully. The main crossing today is Skouw–Wutung, on the coast east of Jayapura.
Timor: 253 km in Two Pieces with Timor-Leste
Indonesia's shortest land border is also its most intricate. The 253 km boundary with Timor-Leste comes in two non-contiguous sections. The larger one cuts Timor in two, running from the Savu Sea on the north coast to the Timor Sea on the south, mostly along rivers: the Talu, the Malibacu, the Tafara and the Massin. The second section wraps around Oecusse, an exclave of Timor-Leste on the north coast of Indonesian West Timor, following the Noel Besi, Ekan, Sonau and Bilomi rivers.
The division goes back to the rivalry between the Portuguese, who settled at Lifau in western Timor in 1556, and the Dutch, who occupied the west of the island from the 1640s. The Treaty of Lisbon of 1859 was the first attempt to fix a precise line, and a further convention in 1893 set up a joint commission, but disputes over the Oecusse boundary continued. The Permanent Court of Arbitration in The Hague settled the matter with an award dated 25 June 1914, delivered by the Swiss judge Charles Édouard Lardy, and boundary posts went up in 1915.
That colonial line became an international border only after Indonesia's 24-year occupation of East Timor, the 1999 referendum in which 78.5% voted for independence, and independence on 20 May 2002. An interim agreement of 2005 fixed about 96% of the land border with 907 coordinate points. The remaining segments, at Noel Besi–Citrana, Bidjael Sunan–Oben and Dilumil–Memo, took far longer: Dilumil–Memo was settled in 2013 and an agreement in principle on the rest was announced in July 2019. Even then the farming village of Naktuka, in the Citrana triangle on the Oecusse border, stayed contentious. Posts placed there in November 2023 drew objections in January 2024 from FONGTIL, the umbrella body of Timor-Leste's non-governmental organisations, and the signing of a final border treaty was postponed. Crossings operate at Mota'ain–Batugade on the north coast and Motamasin–Salele on the south.
2,958 km or 3,108 km? Measuring Indonesia's Land Frontier
Two sets of figures circulate for these borders, and both appear on Wikipedia. The per-border articles give 1,881 km for Malaysia, 824 km for Papua New Guinea and 253 km for Timor-Leste. Indonesia's own boundary figures, summarised in the article on the borders of Indonesia, give 2,019.5 km, 820 km and 268.8 km, for a total of 3,108.3 km. The gap reflects measuring method, not disagreement about where the line lies.
Ten Countries Across the Water: Indonesia’s Maritime Boundaries
Most of Indonesia's frontier is water, negotiated piece by piece since 1969. Roughly clockwise from the north-west:
- India: Indonesia's northernmost waters off Sumatra meet those of India's Andaman and Nicobar Islands. A trijunction with India and Thailand was fixed by a tripartite agreement on 22 June 1978.
- Thailand: continental shelf agreements in the northern Strait of Malacca and the Andaman Sea were signed on 17 December 1971 and 11 December 1975.
- Malaysia: in the Strait of Malacca the line follows the median between the two coasts, set by a continental shelf treaty of 27 October 1969 and a territorial sea treaty of 17 March 1970.
- Singapore: a territorial sea boundary of 24.55 nautical miles (45.47 km) in the Singapore Strait was agreed on 25 May 1973. A 2009 treaty extended it 12.1 km westward.
- Vietnam: a continental shelf boundary in the South China Sea was signed in Hanoi on 26 June 2003. In December 2022 the two governments announced they had concluded talks on the line between their exclusive economic zones, long a source of fishing clashes.
- The Philippines: a 2014 agreement fixed their exclusive economic zone boundary with eight coordinate points, running 1,162.2 km across the Celebes Sea to the Philippine Sea.
- Palau: Indonesia's waters north of Papua meet those of Palau in the Pacific.
- Papua New Guinea: the sea boundary continues the land line into the Torres Strait and the Pacific.
- Australia: a boundary in three segments, across the Arafura and Timor seas and between Java and Christmas Island. Seabed treaties of 18 May 1971 and 9 October 1972 drew the line on the principle of "natural prolongation" of the continental shelf, which placed it well north of the median line. A 1997 treaty extending it was signed in Perth but never ratified.
- Timor-Leste: talks on the sea boundary, which began in 2015, were still under way in 2022.
Sipadan, Ambalat and Natuna: Where the Sea Is Still Contested
The Celebes Sea off eastern Borneo is the clearest example of a colonial treaty that drew the land line and left the water undefined. Indonesia and Malaysia both claimed the small islands of Sipadan and Ligitan from 1969, and in 1998 they agreed to send the case to the International Court of Justice. The court ruled in 2002 that both islands belong to Malaysia. That decision did not draw a maritime boundary, and the two countries still have overlapping claims to the Ambalat seabed blocks east of North Kalimantan. The quarrel dates from a Malaysian map of 1979 that drew its boundary south-east from Sebatik, which Indonesia rejects; during the Sipadan case Indonesia argued that the 4°10′ North line should simply continue out to sea, an argument the court did not accept.
Further north, Indonesia does not consider China a neighbour in any legal sense, but its 200-nautical-mile exclusive economic zone around the Natuna Islands, a 272-island group in the Riau Islands province, is overlapped by China's nine-dash line. After Chinese fishing boats escorted by a coast guard vessel entered the zone in late 2019, Jakarta lodged a formal protest and sent around 600 troops and eight warships, and President Joko Widodo visited the islands in January 2020. In November 2024 the two governments signed a memorandum on "joint maritime development" in areas of overlapping claims. Indonesia's foreign ministry said the memorandum did not affect its sovereign rights and that the Chinese claim has no legal basis.
Seen together, Indonesia's land borders are short, colonial and mostly settled, while its sea boundaries are long, modern and in places still open. The pattern is mirrored from the other side of Borneo in our guide to the countries that border Malaysia, and the islands that generate all those maritime lines are mapped one by one in our complete guide to the Indonesian islands.