How a 7,250 square kilometre wedge of sea became a shared resource for two nations

Countries & Cities · Published 15 October 2026 Swipe more like this →

In short: The border between Malaysia and Thailand is defined by mountain watersheds and a winding river, but its most fascinating feature lies offshore. Rather than fighting over overlapping claims in the Gulf of Thailand, both nations established a unique joint development area to share the region's valuable mineral resources.

The 658-kilometre land boundary between Malaysia and Thailand traces its origins to the 1909 Anglo-Siamese Treaty. This agreement formalised the division between British-influenced Malay states and Siamese territories, establishing a border that runs along mountain watersheds and the thalweg of the Golok River. While the land border is largely settled, a small 8.5-kilometre stretch at Jeli Hill remains disputed, covering 42 hectares. This disagreement emerged in the 1990s as demarcation work neared completion, with officials noting that the physical landscape had shifted since the original 1909 protocol was drafted.

Maritime boundaries present a more complex challenge. In the Gulf of Thailand, both countries hold overlapping claims over the continental shelf, largely due to differing interpretations of baselines used to calculate equidistant lines. Rather than allowing these sovereignty disputes to escalate, Malaysia and Thailand signed a memorandum of understanding in 1979 and a formal agreement in 1990 to create a joint development area. This 7,250 square kilometre wedge allows both nations to exploit non-living natural resources on a 50:50 basis without formally relinquishing their respective sovereignty claims.

This cooperative model has even expanded to include third parties. A small portion of the disputed area also involves Vietnam, and in 1999, all three countries agreed to apply the same joint development principles to that overlapping zone. These agreements serve as a pragmatic, interim solution that prioritises economic cooperation over territorial deadlock. While the border has seen historical shifts—including during World War II when Japan briefly transferred control of several Malay states to Thailand—the modern era is defined by these sophisticated diplomatic frameworks that manage resources in the absence of a final border settlement.

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