Cambodia's UNCLOS ratification unites ASEAN in law, not in politics

Summary

In a commentary published on 25 September 2026 on Fulcrum, the platform of Singapore's ISEAS, Huynh Trung Dung and Chheang Vannarith examine Cambodia's ratification of the UN Convention on the Law of the Sea (UNCLOS, the 1982 treaty governing maritime zones). Cambodia signed the convention in 1983 but only ratified it in January 2026, becoming the 172nd party and the last ASEAN member to join. The authors see the deteriorating relationship with Thailand as the main driver. On 5 May 2026 Thailand terminated a 2001 memorandum of understanding on overlapping continental shelf claims. On 2 June Cambodia launched compulsory conciliation under Annex V of the convention; the first meeting took place in Singapore on 14–16 September.

The economic stakes are large. According to one report, the overlapping area of about 26,000 km² in the Gulf of Thailand is estimated to hold 300 billion $ worth of oil and gas. The authors argue that with every ASEAN member now bound by the same legal framework, the bloc could gain leverage in Code of Conduct negotiations with China. They recall 2 precedents, however. In 2012 ASEAN foreign ministers failed to issue a joint communiqué for the first time, and in 2016 Phnom Penh blocked any ASEAN text referring to the South China Sea arbitration ruling. Their conclusion is cautious: ratification does not change the game, and its practical effect depends on Phnom Penh's calculations towards Beijing. They recommend swiftly making operational the ASEAN Maritime Centre announced in May 2026.