タイ族が11世紀ごろに現在のタイへ下って来た当時は、タイ族はピー信仰（精霊信仰）を享受していたが、上座部仏教が最大の勢力を持つ宗教として成立するのはラームカムヘーン王（在位・1279年? - 1300年?）の時代である。後に王に即位したリタイ王（在位1347年? - 1368年?）は、衰えて行くスコータイ王朝を仏教思想で立て直そうと、タイ族の君主として初めて出家を行い、タンマラーチャー（仏法王）と名乗った。これは大仏を建てることで、天皇の権威を高め国政を安定化しようとした聖武天皇のケースと似ている。リタイの出家、及びタンマラーチャーの思想は、王権を高める上で非常に有利であったためアユタヤ王朝、ラーンナータイ王朝などの周辺諸王国に伝播していった。さらに、この出家の習慣は初期は王が行っていたが、後には民衆にも伝播し、タイ族の男子は成人すると必ず出家すると言うのが暗黙の義務になっていった。
1.1 Early traditions Some scholars believe that Buddhism must have been flowing into Thailand from India at the time of the Indian emperor Ashoka of the Maurya Empire and into the first millennium after Christ. During the 5th to 13th centuries, Southeast Asian empires were influenced directly from India and followed Mahayana Buddhism. The Chinese pilgrim Yijing noted in his travels that in these areas, all major sects of Indian Buddhism flourished. Srivijaya to the south and the Khmer Empire to the north competed for influence and their art expressed the rich Mahāyāna pantheon of bodhisattvas. From the 9th to the 13th centuries, the Mahāyāna and Hindu Khmer Empire dominated much of the Southeast Asian peninsula. Under the Khmer Empire, more than 900 temples were built in Cambodia and in neighboring Thailand.
After the decline of Buddhism in India, missions of Sinhalese monks gradually converted the Mon people and the Pyu city-states from Ari Buddhism to Theravāda and over the next two centuries also brought Theravāda Buddhism to the Bamar people, Thailand, Laos and Cambodia, where it supplanted previous forms of Buddhism. Theravada Buddhism was made the state religion only with the establishment of the Sukhothai Kingdom in the 13th century.
1.2 13th–19th centuries The details of the history of Buddhism in Thailand from the 13th to the 19th century are obscure, in part because few historical records or religious texts survived the Burmese destruction of Ayutthaya, the capital city of the kingdom, in 1767. Ayutthaya was the center of Thai Tantric Theravada, which included the Yogāvacara tradition, and has survived in the contemporary Dhammakaya Movement. The Tantric Buddhist Yogāvacara tradition was a mainstream Buddhist tradition in Cambodia, Laos and Thailand well into the modern era. An inscription from northern Thailand with tantric elements has been dated to the Sukhothai Kingdom of the 16th century. Kate Crosby notes that this attestation makes the tantric tradition earlier than "any other living meditation tradition in the contemporary Theravada world," predating the popular "New Burmese Satipatthana Method", better known as Vipassana meditation.
The anthropologist-historian S. J. Tambiah, however, has suggested a general pattern for that era, at least with respect to the relations between Buddhism and the sangha on the one hand and the king on the other hand. In Thailand, as in other Theravada Buddhist kingdoms, the king was in principle thought of as patron and protector of the religion (sasana) and the sangha, while sasana and the sangha were considered in turn the treasures of the polity and the signs of its legitimacy. Religion and polity, however, remained separate domains, and in ordinary times the organizational links between the sangha and the king were not close.
Replica of Ashok pillar at Wat Umong in Chiang Mai, Thailand, 13th century. Shows the establishment of Buddhism by Lanna Dynasty's King Mangrai in northern Thailand
Among the chief characteristics of Thai kingdoms and principalities in the centuries before 1800 were the tendency to expand and contract, problems of succession, and the changing scope of the king's authority. In effect, some Thai kings had greater power over larger territories, others less, and almost invariably a king who sought successfully to expand his power also exercised greater control over the sangha. That control was coupled with greater support and patronage of the ecclesiastical hierarchy. When a king was weak, however, protection and supervision of the sangha also weakened, and the sangha declined. This fluctuating pattern appears to have continued until the emergence of the Chakri Dynasty in the last quarter of the 18th century.
1.3 Modern era
Buddhist monk chants paritta to a group of Siamese women in 1900. By the 19th century, and especially with the coming to power in 1851 of King Mongkut, who had been a monk himself for twenty-seven years, the sangha, like the kingdom, became steadily more centralized and hierarchical in nature and its links to the state more institutionalized. As a monk, Mongkut was a distinguished scholar of Pali Buddhist scripture. Moreover, at that time the immigration of numbers of monks from Burma was introducing the more rigorous discipline characteristic of the Mon sangha. Influenced by the Mon and guided by his own understanding of the Tipitaka, Mongkut began a reform movement that later became the basis for the Dhammayuttika order of monks. Under the reform, all practices having no authority other than custom were to be abandoned, canonical regulations were to be followed not mechanically but in spirit, and acts intended to improve an individual's standing on the road to nirvana but having no social value were rejected. This more rigorous discipline was adopted in its entirety by only a small minority of monasteries and monks. The Mahanikaya order, perhaps somewhat influenced by Mongkut's reforms but with a less exacting discipline than the Dhammayuttika order, comprised about 95 percent of all monks in 1970 and probably about the same percentage in the late 1980s. In any case, Mongkut was in a position to regularize and tighten the relations between monarchy and sangha at a time when the monarchy was expanding its control over the country in general and developing the kind of bureaucracy necessary to such control. The administrative and sangha reforms that Mongkut started were continued by his successor. In 1902 King Chulalongkorn (Rama V, 1868–1910) made the new sangha hierarchy formal and permanent through the Sangha Law of 1902, which remained the foundation of sangha administration in modern Thailand. While Buddhism in Thailand remained under state centralization in the modern era, Buddhism experienced periods of tight state control and periods of liberalization depending on the government at the time.
1.3.1 Statistics Approximately 94 percent of Thailand's population is Buddhist (five percent Muslim). As of 2016 Thailand had 39,883 wats (temples). Three hundred-ten are royal wats, the remainder are private (public). There were 298,580 Buddhist monks, 264,442 of the Maha Nikaya order and 34,138 of the Dhammayuttika Nikaya order. There were 59,587 Buddhist novice monks.
- 羅馬發音：Namo Tassa， Bagavato， Arahato， Samma Sambhudhassa
- 慧明《圖解：一次完全讀懂佛經（二版）》 页面存档备份，存于互联网档案馆：西元13世紀中葉，素可泰王朝的國王蘭摩甘亨派人請斯里蘭卡的僧侶前來泰國傳播上座部佛教
- 《佛教·佛经故事》 页面存档备份，存于互联网档案馆：佛教由斯里兰卡传入泰国约在12世纪左右。13世纪，泰国素可泰王朝宣布奉佛教为国教。
- 中國佛教協會, 佛教將成為連接中國與東盟之間友誼的橋樑, [2019-03-05], （原始内容存档于2015-08-23）：“至於單個國家，人口佛教信徒比例最高的是柬埔寨，占人口總數的93%，在泰國，佔92%;在緬甸，佔82%;在老撾，佔77%;在越南，佔76%;在新加坡，佔40%;在馬來西亞，佔20%;在文萊，佔13.5%。”
- 《中國佛教史·三、泰国佛教的国教化》 页面存档备份，存于互联网档案馆，任继愈