Cambodia’s Ratification of the UN Convention on the Law of the Sea and ASEAN’s Maritime Cohesion

More than four decades after signing the United Nations Convention on the Law of the Sea (UNCLOS) in 1983, Cambodia’s National Assembly finally ratified the treaty in January 2026, becoming the 172nd state party and the last ASEAN member state (AMS) to do so. The ratification has carried significant implications for Cambodia and ASEAN.

Cambodia’s perception of an existential threat posed by Thailand, compounded by growing uncertainty over the management of the maritime boundary and overlapping claims between the two countries, was a major factor driving Cambodia to ratify UNCLOS. The ratification empowers the government to resolve Cambodia’s maritime differences with neighbours through legal and other processes, consistent with its stated commitment to principle-based foreign policy and peaceful dispute settlement.

As Cambodia-Thailand relations had soured over the year, Thailand’s National Security Council approved the revocation of the Memorandum of Understanding (MoU) 44, signed in 2001 by both countries on the area of their overlapping maritime claims to the continental shelf, on 23 April 2026. The Thai Cabinet terminated the MoU on 5 May 2026. In response, Cambodia notified Thailand and the UN Secretary-General on 2 June 2026 that it was invoking compulsory conciliation under Annex V of UNCLOS. Cambodia’s Prime Minister Hun Manet framed this carefully, saying that “this is not unilateral action” but “an effort to resolve the dispute peacefully, through international law, and in good faith”. The first conciliation meeting between the two countries took place in Singapore from 14 to 16 September 2026.

The economic stakes are substantial for Cambodia: the country has a long coastline (more than 400 kilometres in length). Under Part V of UNCLOS, Cambodia has “sovereign rights for the purpose of exploring and exploiting, conserving and managing the natural resources of the waters superjacent to the seabed and of the seabed and its subsoil, and with regard to other activities for the economic exploitation and exploration of its exclusive economic zone (EEZ), such as the production of energy from the water, currents and winds”. Importantly, the Overlapping Claims Area in the Gulf of Thailand spans some 26,000 square kilometres and is thought to hold significant oil and gas, valued by at least one report at around USD300 billion.

Beyond the dispute with Thailand, ratifying UNCLOS strengthens Cambodia’s footing in regional maritime affairs, including the potential enhancement of maritime cooperation with other AMS. Minister of Information Neth Pheaktra described UNCLOS as a “comprehensive legal basis” for navigation, fisheries, seabed resources and environmental protection. Ratification brings obligations and legal standing for Cambodia (under Part XII) on the protection of the marine environment, potential participation in the institutions the Convention created, including the International Seabed Authority (ISBA) and the International Tribunal for the Law of the Sea (ITLOS) through the right of UNCLOS parties to stand for election to them. The ratification connects Cambodia to the framework on regional cooperation and capacity building on illegal, unreported and unregulated fishing.

The case for tackling transnational crime is equally practical, as UNCLOS sets out jurisdictional rules for boarding, search and rescue, pursuit and cooperation against trafficking at sea. By ratifying UNCLOS, Cambodia signals commitment to a legal framework within which joint patrols, information sharing and interdiction agreements can be negotiated.

For ASEAN, Cambodia’s ratification carries meaningful significance, as now all AMS are parties to UNCLOS, underscoring ASEAN’s respect for and cohesion around international law and global maritime governance. In principle, ASEAN can be better positioned to stand together on future maritime governance, with every member bound by the same legal framework, reaffirming its “shared commitment to upholding international law, in particular…UNCLOS”, as declared by its leaders in May 2026. Cambodia’s ratification lays the legal ground for all AMS to resolve or manage their maritime disputes through UNCLOS, which could help mitigate tensions, and enhance trust and transparency in the region.

Whether all this translates into practice still depends on Phnom Penh’s political calculus, particularly vis-à-vis Beijing.

Externally, the ratification could strengthen ASEAN’s position and leverage on UNCLOS-related affairs when the bloc deals with outside partners, such as the Code of Conduct (CoC) negotiations with China. This is critically important for ASEAN in its quest for consensus on sensitive, security-related issues such as marine scientific research, undersea cables and seabed minerals within EEZs. Such a position was tested in 2012, when ASEAN foreign ministers failed to reach consensus amid disagreements over how to characterise developments in the South China Sea, including references to specific disputes and UNCLOS’ application. This resulted in an unprecedented failure to issue a Joint Communiqué at the conclusion of the Foreign Ministers’ Meeting. Four years later, Phnom Penh did not support the 2016 South China Sea Arbitral Award issued under UNCLOS, thus blocking any ASEAN joint declaration or statement that would support or even mention the 2016 ruling.

Policy Implications

Whether all this translates into practice still depends on Phnom Penh’s political calculus, particularly vis-à-vis Beijing. Cambodia’s ratification does not mean that all maritime differences between Phnom Penh and other AMS will be resolved. Yet ASEAN may leverage its members’ now-universal UNCLOS membership to narrow internal gaps on maritime issues and to advance intra-regional and external cooperation on maritime development initiatives in Southeast Asia. One path would be to accelerate the establishment and operationalisation of the ASEAN Maritime Centre, announced in May 2026. While Cambodia’s ratification of UNCLOS is not a game changer, it is a good opportunity for Cambodia — and ASEAN — to take a small step towards enhancing a rules-based regional maritime order.

2026/263