Thailand ( TY-land, TY-lənd), historically known as Siam (), officially the Kingdom of Thailand, is a country in Southeast Asia spanning 513,120 square kilometres (198,120 sq mi), with a population of almost 70 million. It is bordered to the north by Myanmar and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the west by the Andaman Sea and Myanmar. Thailand also shares maritime borders with Vietnam to the southeast, and Indonesia and India to the southwest. Thailand has experienced multiple coups and military dictatorships. From 2014 to 2019 it was formally under military rule, until the military introduced a new constitution and held elections which established the framework of a parliamentary constitutional monarchy. In practice, however, structural advantages in the constitution have ensured the military's hold on power. Bangkok is the nation's capital and largest city.

Tai peoples migrated from southwestern China to mainland Southeast Asia from the 11th century. Indianised kingdoms such as the Mon, Khmer Empire and Malay states ruled the region, competing with Thai states such as the Kingdoms of Ngoenyang, Sukhothai, Lan Na and Ayutthaya, which also rivalled each other. European contact began in 1511 with a Portuguese diplomatic mission to Ayutthaya, which became a regional power by the end of the 15th century. Ayutthaya reached its peak during cosmopolitan Narai's reign, gradually declining thereafter until being ultimately destroyed in the Burmese–Siamese War. Taksin quickly reunified the fragmented territory and established the short-lived Thonburi Kingdom. He was succeeded in 1782 by Buddha Yodfa Chulaloke, the first monarch of the current Chakri dynasty.

Throughout the era of Western imperialism in Asia, Siam remained the only nation in the region to avoid being colonised by foreign powers, although it was often forced to cede both territory and trade concessions in unequal treaties. The Siamese system of government was centralised and transformed into a modern unitary absolute monarchy in the reign of Chulalongkorn. In World War I, Siam sided with the Allies, a political decision made in order to amend the unequal treaties. Following a bloodless revolution in 1932, it became a constitutional monarchy and changed its official name to Thailand, which was an ally of Japan in World War II. In the late 1950s, a military coup under Field Marshal Sarit Thanarat revived the monarchy's historically influential role in politics. Thailand became a major ally of the United States, and played an anti-communist role in the region as a member of the failed SEATO, but since 1975, had sought to improve relations with Communist China and Thailand's neighbors. Apart from a brief period of parliamentary democracy in the mid-1970s, Thailand has periodically alternated between democracy and military rule. Since the 2000s, it has been caught in a series of bitter political conflict between supporters and opponents of Thaksin Shinawatra, which culminated in two coups, in 2014, and the establishment of its current constitution, and ongoing pro-democracy protests.

Thailand is a middle power in global affairs, and a founding member of ASEAN; ranking high in the Human Development Index. It has the second-largest economy in Southeast Asia, and the 22nd-largest in the world by PPP. Thailand is classified as a newly industrialised economy; manufacturing, agriculture, and tourism are leading sectors of the economy.