Ayutthaya (Thai: อาณาจักรอยุธยา, RTGS: Anachak Ayutthaya, also Ayudhya, [ʔaːnaːtɕ͡àk ʔajúttʰajaː]) was a Siamese kingdom that existed from 1350 to 1767. Ayutthaya was friendly towards foreign traders, including the Chinese, Vietnamese (Annamese), Indians, Japanese and Persians, and later the Portuguese, Spanish, Dutch and French, permitting them to set up villages outside the walls of the capital, also called Ayutthaya. In the sixteenth century, it was described by foreign traders as one of the biggest and wealthiest cities in the East. The court of King Narai (1656–88) had strong links with that of King Louis XIV of France, whose ambassadors compared the city in size and wealth to Paris.

By 1550, the kingdom's vassals included some city-states in the Malay Peninsula, Sukhothai, and parts of Cambodia.

In foreign accounts, Ayutthaya was called Siam, but many sources say the people of Ayutthaya called themselves Tai, and their kingdom Krung Tai, 'Tai Kingdom'.

According to the most widely accepted version of its origin, the Siamese state based at Ayutthaya in the valley of the Chao Phraya River rose from the earlier, nearby kingdoms of Lavo (at that time still under the Khmer control) and Suphannaphoom (Suvarnabhumi). One source says that, in the mid-fourteenth century, due to the threat of an epidemic, King U Thong moved his court south into the rich floodplain of the Chao Phraya on an island surrounded by rivers, which was the former seaport city of Ayothaya, or Ayothaya Si Raam Thep Nakhon, the Angelic City of Sri Rama. The new city was known as Ayothaya, or Krung Thep Dvaravadi Si Ayothaya. Later it became widely known as Ayutthaya, the Invincible City.