Thais date the founding of their nation to the 13th century. According to tradition, Thai chieftains gained independence from the Khmer Empire at Sukhothai, which was established as a sovereign Kingdom by Pho Khun Si Indrathit in 1238. A political feature called, in Thai, 'father governs children' existed at this time. Everybody could bring their problems to the king directly; there was a bell in front of the palace for this purpose. The city briefly dominated the area under King Ramkhamhaeng, who established the Thai alphabet, but after his death in 1365 it fell into decline and became subject to another emerging Thai state known as the Ayutthaya kingdom, which dominated southern and central Thailand until the 1700s.

The first ruler of the Kingdom of Ayutthaya, King Ramathibodi I, made two important contributions to Thai history: the establishment and promotion of Theravada Buddhism as the official religion — to differentiate his kingdom from the neighbouring Hindu kingdom of Angkor — and the compilation of the Dharmashastra, a legal code based on Hindu sources and traditional Thai custom. The Dharmashastra remained a tool of Thai law until late in the 19th century. Beginning with the Portuguese in the 16th century, Ayutthaya had some contact with the West, but until the 1800s, its relations with neighboring nations as well as with India and China, were of primary importance. Ayutthaya dominated a considerable area, ranging from the Islamic states on the Malay Peninsula to states in northern Thailand. Nonetheless, the Burmese, who had control of Lanna and had also unified their kingdom under a powerful dynasty, launched several invasion attempts in the 1750s and 1760s. Finally, in 1767, the Burmese attacked the capital city and conquered it. The royal family fled the city where the king died of starvation ten days later. The Ayutthaya royal line had been extinguished. Overall there are 33 kings in this period, including an unofficial king.

After more than 400 years of power, in 1767, the Kingdom of Ayutthaya was brought down by invading Burmese armies, its capital burned, and the territory split. General Taksin managed to reunite the Thai kingdom from his new capital of Thonburi and declared himself king in 1769. However, Taksin allegedly became mad, and he was deposed, taken prisoner, and executed in 1782. General Chakri succeeded him in 1782 as Rama I, the first king of the Chakri dynasty. In the same year he founded the new capital city at Bangkok, across the Chao Phraya river from Thonburi, Taksin's capital. In the 1790s Burma was defeated and driven out of Siam, as it was then called. Lanna also became free of Burmese occupation, but the king of a new dynasty was installed in the 1790s was effectively a puppet ruler of the Chakri monarch.

The heirs of Rama I became increasingly concerned with the threat of European colonialism after British victories in neighboring Burma in 1826. The first Thai recognition of Western power in the region was the Treaty of Amity and Commerce with the United Kingdom in 1826. In 1833, the United States began diplomatic exchanges with Siam, as Thailand was called until 1939, and again between 1945 and 1949. However, it was during the later reigns of King Chulalongkorn, and his father King Mongkut, that Thailand established firm rapprochement with Western powers. It is a widely held view in Thailand that the diplomatic skills of these monarchs, combined with the modernising reforms of the Thai Government, made Siam the only country in South and Southeast Asia to avoid European colonisation. This is reflected in the country's modern name, Prathet Thai or Thai‐land, used unofficially between 1939 and 1945 and officially declared on May 11, 1949, in which prathet means "nation" and thai means "free".