The Gulf of Thailand, also known as the Gulf of Siam, is a shallow inlet in the southwestern South China Sea, bounded between the southwestern shores of the Indochinese Peninsula and the northern half of the Malay Peninsula. It is around 800 km (500 mi) in length and up to 560 km (350 mi) in width, and has a surface area of 320,000 km2 (120,000 sq mi). The gulf is surrounded on the north, west and southwest by the coastlines of Thailand, on the northeast by Cambodia and the Mekong Delta region of Vietnam, and opens to the South China Sea in the southeast.

The South China Sea is a marginal sea of the Western Pacific Ocean. It is bounded in the north by South China, in the west by the Indochinese Peninsula, in the east by the islands of Taiwan and northwestern Philippines, and in the south by the Indonesian islands of Borneo, eastern Sumatra and the Bangka Belitung Islands, encompassing an area of around 3,500,000 km2 (1,400,000 sq mi). It communicates with the East China Sea via the Taiwan Strait, the Philippine Sea via the Luzon Strait, the Sulu Sea via the straits around Palawan, and the Java Sea via the Karimata and Bangka Straits. The Gulf of Thailand and the Gulf of Tonkin are part of the South China Sea.