The Mekong forms long stretches of the border between Thailand and Laos. In much of the frontier, Thailand lies west of the river and Laos lies east, although the boundary also follows land and smaller rivers in places.
The Thailand–Laos border is about 1,845 kilometres long, and more than half of it follows the Mekong. The river-border section begins near the tripoint with Myanmar in the north, then continues southeast before the boundary departs from the Mekong near the southern part of Laos.
The Mekong is one of mainland Southeast Asia’s defining waterways. It rises on the Tibetan Plateau and flows through or along China, Myanmar, Laos, Thailand, Cambodia, and Vietnam before reaching the South China Sea. Its floodplain and seasonal flow support fisheries, farming, transport, and dense riverfront settlements.
The Mekong is not the entire Thailand–Laos boundary. The frontier also follows stretches near the Hueang River and overland ridges, including the Dângrêk Mountains. Another common confusion is the Salween, which forms parts of other regional boundaries but is not the principal river dividing Thailand and Laos.