The Gulf of Thailand is a shallow arm of the South China Sea covering roughly 320,000 km², bordered by Thailand, Cambodia, Vietnam and the eastern coast of Peninsular Malaysia. Its average depth is only about 45 metres and its deepest point is around 80 metres — shallower than many lakes, and the single fact that governs its tides, its fisheries, its oil industry and the islands that sit in it.
A Drowned River Plain, Not an Ocean Basin
The gulf is geologically young. During the last glacial maximum, around 20,000 years ago, sea level stood roughly 120 metres lower and the entire gulf floor was dry land — part of the Sundaland shelf that joined the Malay Peninsula, Sumatra, Java and Borneo into one continuous landmass. The Chao Phraya and the other rivers now emptying into the gulf ran across that plain to a coastline far to the southeast.
Rising seas flooded it between roughly 14,000 and 6,000 years ago. That is why the gulf is a shallow dish rather than a basin with a deep axis, why its sediment is river-derived rather than oceanic, and why it has no trench, no subduction zone and effectively no seismic hazard of its own. Thailand's Gulf coast was untouched by the 2004 Indian Ocean tsunami, which devastated its Andaman coast on the other side of the peninsula — the peninsula itself blocked it.
The gulf opens to the South China Sea across a line usually drawn from Cape Cà Mau at the southern tip of Vietnam to Kota Bharu in Malaysia. Within that, the northern end forms the Bight of Bangkok, the innermost recess where the Chao Phraya, Mae Klong, Tha Chin and Bang Pakong rivers all discharge within a short stretch of coast.
Where the Water Comes From and How It Moves
The rivers do more than deliver sediment. Their freshwater input strongly stratifies the upper gulf, producing surface salinity well below normal seawater in the wet season and contributing to the seasonal low-oxygen conditions that periodically kill fish in the Bight of Bangkok.
Circulation runs on the monsoon. The northeast monsoon from roughly November to March drives water into the gulf and generates a counter-clockwise surface circulation; the southwest monsoon from about May to September reverses much of it. Because the gulf is shallow and nearly enclosed, flushing is slow — water residence times are measured in months to years, which means pollutants and nutrients delivered by the rivers stay in the system rather than dispersing.
Tides are unusual and locally notorious. Much of the gulf experiences a diurnal tide — one high and one low per day rather than the usual two — because the basin's dimensions resonate with the diurnal tidal constituents rather than the semi-diurnal ones. The range is modest, typically under two metres, but the single-cycle pattern affects everything from fishing schedules to port operations.
Oil, Gas and the Boundaries They Forced
Gas was found under the gulf in the 1970s and the Erawan field began producing in 1981, transforming Thailand's energy position — domestic gas from the gulf has supplied a large share of the country's electricity generation for decades. Oil is produced too, in smaller quantities.
Hydrocarbons under a semi-enclosed sea bordered by four countries created predictable disputes, and the responses have been unusually pragmatic:
- Thailand–Malaysia established a Joint Development Area in 1979 over an overlapping claim, with production shared equally rather than the boundary litigated — one of the earliest arrangements of its kind and a frequently cited model.
- Thailand–Vietnam agreed a maritime boundary in 1997, settling their overlap outright.
- Malaysia–Vietnam operate a separate joint arrangement over their own overlapping zone.
- Thailand–Cambodia have an Overlapping Claims Area of roughly 26,000 km² that remains unresolved. Successive memoranda of understanding, including one signed in 2001, have not produced development, and the dispute is entangled with Thai domestic politics.
The contrast with the wider South China Sea is instructive. The gulf's disputes involve four states with broadly comparable leverage and no strategic chokepoint at stake, so joint development was achievable. Further east, where claims overlap and the stakes include sea lanes, that has not been the case.
Reefs, Whale Sharks and a Fishery Under Strain
The gulf's warm shallow water supports coral reefs, seagrass meadows and mangrove along much of its coast. Bryde's whales feed in the upper gulf and are regularly observed there; whale sharks appear around the islands; dugong persist in seagrass beds, particularly on the Andaman side but in the gulf as well. Green and hawksbill turtles nest on gulf beaches.
The pressures are severe and well documented. Thailand's trawl fishery expanded rapidly from the 1960s, and catch per unit effort in the gulf fell by roughly an order of magnitude over the following decades — the standard textbook illustration of a tropical multi-species fishery being fished down. A large proportion of the catch is now low-value "trash fish" processed into fishmeal for aquaculture rather than eaten directly.
Mangrove loss has been heavy, much of it converted to shrimp ponds in the 1980s and 1990s, removing both nursery habitat and coastal protection. Coastal erosion in the upper gulf is acute: parts of the Samut Prakan and Samut Sakhon shoreline have retreated by hundreds of metres, driven by mangrove clearance, upstream dams cutting the Chao Phraya's sediment supply, groundwater extraction causing Bangkok to subside, and rising sea level acting together.
The Islands, and Why They Differ From the Andaman Side
The gulf's best-known islands sit off the Surat Thani coast — Koh Samui, Koh Phangan and the diving centre of Koh Tao — with Koh Chang and Koh Kut near the Cambodian border, and Koh Samet close to Bangkok.
They are granite islands, rounded and boulder-strewn, with sandy bays. That is the clearest physical contrast with the Andaman Sea side of the peninsula, where Phuket, Krabi and Phang Nga Bay are karst — limestone towers rising vertically out of the water, with caves, arches and sea stacks. The two coasts look nothing alike because they are different rock.
The seasons are also opposed, which is the practically useful part. The Andaman coast is wettest during the southwest monsoon, roughly May to October. The gulf islands are driest then, and take their heaviest rain during the northeast monsoon in November and December, when Koh Samui and Koh Phangan can see serious storms. A traveller in either half of the year can find dry weather on one side of the peninsula.
Ports, Sea Lanes and the Canal That Was Never Cut
The gulf is Thailand's commercial front door. Laem Chabang, opened in the early 1990s southeast of Bangkok, is the country's principal container port and among the busiest in Southeast Asia; Map Ta Phut nearby serves the Eastern Seaboard petrochemical complex that gulf gas made possible. Klong Toey, the old river port inside Bangkok, remains in use but is limited by draught on the Chao Phraya.
Cambodia's only deep-water port, Sihanoukville, sits on the gulf's eastern shore, and Vietnam's Phú Quốc island lies in the northeastern corner close to the Cambodian coast — an arrangement that has been a point of friction between the two countries. Malaysia's gulf coast, facing Kelantan and Terengganu, is the quieter side, more fishing and gas than container traffic.
Running through all of this is a proposal that has been raised repeatedly since the seventeenth century: a canal across the Isthmus of Kra, at its narrowest around 44 kilometres, linking the gulf to the Andaman Sea and letting shipping bypass the Strait of Malacca. It has been studied by Thai governments, by French engineers in the colonial era and by various investor consortia since, and has never been built — the cost is enormous, the engineering across hilly terrain is difficult, Singapore's transhipment economy has an obvious interest against it, and the route cuts through provinces where Thai governments have long-standing security concerns. A land bridge of rail and pipeline between two new ports has been floated as the cheaper alternative, without so far progressing either.
The Gulf Against the Andaman Sea and the Persian Gulf
Set beside the Andaman Sea on the other side of Thailand, the gulf is shallower, warmer, calmer and far less biodiverse. The Andaman is an open ocean margin reaching depths over 4,000 metres, connected to the Indian Ocean, seismically active along the Sunda Trench, and with a tidal range several times larger. Its reefs are richer; its water clarity is better; its exposure to tsunami is real.
A closer structural analogue is the Persian Gulf — another shallow, semi-enclosed, hydrocarbon-rich sea averaging around 35 metres deep, bordered by multiple states, with slow flushing and high temperatures. Both are effectively drowned river plains, both have had their marine ecology reshaped by oil development and coastal construction, and both have had to invent boundary arrangements for seabed resources. The differences are climatic: the Persian Gulf is hypersaline because evaporation vastly exceeds river input, while the Gulf of Thailand runs fresher than the open ocean because monsoon rivers pour into it.
That single contrast — evaporation-dominated versus river-dominated — explains most of what separates their ecosystems. The wider regional setting, including the peninsula that divides Thailand's two coasts, is covered in the Thailand country guide.