Myanmar is the largest country in mainland Southeast Asia by area.
Myanmar, also called Burma, covers roughly 676,600 square kilometres. It is larger than Thailand, Vietnam, Laos and Cambodia, the other major mainland states commonly included in this regional grouping. Its territory stretches from the Himalayan foothills in the north to the Andaman Sea and the Bay of Bengal in the south.
The country’s size reflects its broad geographic range: high mountains lie along its borders with India and China, while the central lowlands are shaped by the Irrawaddy River basin. Myanmar also has a long, narrow southeastern extension along the Malay Peninsula, bordering Thailand.
The phrase “mainland Southeast Asia” matters. Southeast Asia is often divided into continental mainland areas and maritime island regions. Malaysia is partly on the mainland and partly on Borneo, so rankings can vary slightly depending on whether a source counts all of Malaysia or only Peninsular Malaysia. That distinction does not change the answer: Myanmar remains substantially larger than every other country wholly or partly included in mainland Southeast Asia.