At 514,000 km? (198,000 sq mi), Thailand is the world's 50th largest country in land mass, whilst it is the world's 28th largest country in terms of population. It is comparable in population to countries such as Iran and Peru, and is similar in land size to France and California in the US; it is just over twice the size of the entire United Kingdom, and 1.4 times the size of Germany.

Thailand is home to several distinct geographic regions, partly corresponding to the provincial groups. The north of the country is mountainous, with the highest point being Doi Inthanon at 2,565 metres above sea level (8,415 ft). The northeast, Isan, (see special section on this region) consists of the Khorat Plateau, bordered to the east by the Mekong river. The centre of the country is dominated by the predominantly flat Chao Phraya river valley, which runs into the Gulf of Thailand. The south consists of the narrow Kra Isthmus that widens into the Malay Peninsula.

The local climate is tropical and characterized by monsoons. There is a rainy, warm, and cloudy southwest monsoon from mid-May to September, as well as a dry, cool northeast monsoon from November to mid-March. The southern isthmus is always hot and humid. Major cities beside the capital Bangkok include Nakhon Ratchasima, Khon Kaen, Udon Thani, Ubon Ratchathani, Nakhon Sawan, Chiang Mai, Phitsanulok, Surat Thani, Phuket and Hat Yai.