At 514,000 km² (198,000 sq mi) , Thailand is the world's 49th-largest country. It is comparable in size to France, and somewhat larger than the US state of California.

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 (8,415 ft). The northeast 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 characterised 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.