Buddhism is said to have first appeared in Thailand among the Mon people in the third century B.C.E. The Mons left China about 2,000 years ago and settled in both Thailand and Burma. They are believed to be the first settlers in Thailand. Once established, they soon encountered other peoples arriving from the north. Many small kingdoms were subsequently established across the land, each vying for power over another. It is likely, considering the first settlers were from China, that some Chinese Buddhist influence was possible. However, Buddhism is generally considered to have appeared in Thailand from India and not from China.

By the thirteenth century, missionaries from Sri Lanka were able to convince the king of Thailand, Ramkhamhaeng, to convert to Buddhism. Pali was established as the religious language of Thailand and Theravada Buddhism firmly took root, where it thrives to this day.

There was a revival of the Theravadan traditions in the nineteenth and twentieth centuries across Southeast Asia. In Burma and Sri Lanka these reforms were part of the independence movement against colonial rule. In Thailand, which retained its independence, reform was initiated by King Rama IV (reign 1851–1868), who was a monk himself for twenty-seven years.