Before Sukhothai
The land that became Thailand had been Buddhist for centuries. The Mon kingdoms of Dvāravatī, from about the 6th century, left Buddha images and Pāli inscriptions, and the Khmer empire brought both Hindu and Mahāyāna forms into the region. Theravāda grew in country that already knew Buddhism well.
Sukhothai and Ayutthaya
An inscription attributed to Ramkhamhaeng of Sukhothai, late in the 13th century, describes inviting forest dwelling monks from the south who had been trained in the Sri Lankan lineage. That lineage, often called the Lankāvaṃsa, spread through the Thai and Lao kingdoms.
Under Ayutthaya, from 1351 to 1767, kingship was described almost entirely in Buddhist terms: merit, the ten virtues of a ruler, and the duty to support and if necessary purify the monastic order. Temple building was enormous, Pāli scholarship strong, and the Three Worlds cosmology text shaped how ordinary people pictured the universe.
Reform and the modern order
After Ayutthaya was destroyed, Rama I convened a council in 1788 to re-establish the canon. Mongkut, who spent 27 years as a monk before becoming king in 1851, founded the Thammayut order on stricter discipline and closer reading of the texts. By the Sangha Act of 1902 the whole order was brought under a national administration, one of the first in the Buddhist world.
Against that centralising current, the forest tradition of the north east, with teachers such as Ajahn Mun and later Ajahn Chah, kept meditation and a simple monastic life at the centre.
Why it matters
Theravāda Buddhism eventually becomes deeply integrated into Thai religious, cultural and political life.
History and tradition
What historians can say
The date and authenticity of the Ramkhamhaeng inscription have been debated for decades, but the Sri Lankan lineage connections are confirmed from several independent directions, including Sri Lankan sources. Dates of reigns, councils and the Sangha Act are documented.