[3][4] The gulf is surrounded on the north, west and southwest by the coastlines of Thailand (hence the name), on the northeast by Cambodia and the Mekong Delta region of Vietnam, and opens to the South China Sea in the southeast.

It is generally identified with the Great Gulf (Latin: Magnus Sinus) known to Greek, Roman, Arab, Persian, and Renaissance cartographers before the influx of Portuguese explorers removed the phantom Dragon Tail peninsula from European world maps in the 16th century.

The term "Gulf of Siam" was widely used in Western cartography and geographical references up until the mid-20th century, reflecting the colonial-era practice of naming regions based on the prevalent local political entities at the time.

"Siam" itself is an exonym, believed to have origins in the Sanskrit word "Syama," which means "dark" or "brown," referring to the skin tone of the region's inhabitants.

[5] The seabed morphology in the central depression of the gulf is characterised by the presence of elongated mounds and ridges arranged parallel to the axis of the basin.

The long-term erosion imposed by currents of stable orientation modifies pockmarks into long runnels and depressions, and ultimately leads to the formation of the large fields of elongated mounds and ridges, as well as the residual outliers of un-eroded mud and clay sheets.

More than half of "land-based plastic waste leakage" into the sea originates from just five countries: China, Indonesia, the Philippines, Thailand, and Vietnam.

[17] The Thai Marine and Coastal Resources Department has noted that at least 300 sea animals on average—60 per cent of which are whales and dolphins—die from eating plastic fishing gear and trash each year.

[16] Filter feeding invertebrates tested off the coast of Chonburi Province showed high levels of microplastics, leading the authors to warn that, "Health risks are possible when people consume these contaminated marine organisms, particularly shellfish.

"[18] Thailand's Pollution Control Department (PCD) estimates that plastic waste in the country is increasing at an annual rate of 12 percent, or around two million tonnes per year.

[19] In late January 2022, a leak in the pipeline operated by the Star Petroleum Refining Public Company Ltd caused a spill of 20 to 50 tonnes across 47 km2 of water, with some oil reaching the coast of Rayong province 20 km away.

[27] A long-standing dispute between Cambodia and Vietnam in the Gulf of Thailand concerns mainly the island of Phú Quốc or Koh Tral in Khmer, which is off the Cambodian coast.