According to many historians, around 228 BC Sohn Uttar Sthavira (one of the royal monks sent by Ashoka the Great) came to Suvarnabhumi (or Suvannabhumi) which some identify with Thailand along with other monks and sacred books.

During the 5th to 13th centuries, the Southeast Asian empires were influenced directly from India, so that these empires essentially followed traditions of Mahāyāna Buddhism. The Chinese pilgrim Yijing noted in his travels that in these areas, all major sects of Indian Buddhism flourished. The Srivijaya Empire 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 Buddhist 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 monks from Sri Lanka gradually converted Burmese Buddhism to Theravāda, and in the next two centuries also brought Theravāda Buddhism to the areas of Thailand, Laos, and Cambodia, where it supplanted previous forms of Buddhism. Theravada Buddhism was made the state religion only with the establishment of the Thai kingdom of Sukhothai in the 13th century AD.

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. 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.

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.

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 1980's. 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.

Three major forces have influenced the development of Buddhism in Thailand. The most visible influence is that of the Theravada school of Buddhism, imported from Sri Lanka. While there are significant local and regional variations, the Theravada school provides most of the major themes of Thai Buddhism. By tradition, Pāli is the language of religion in Thailand. Scriptures are recorded in Pāli, using either the modern Thai script or the older Khom and Tham scripts. Pāli is also used in religious liturgy, despite the fact that most Thais understand very little of this ancient language. The Pāli Tipitaka is the primary religious text of Thailand, though many local texts have been composed in order to summarise the vast number of teachings found in the Tipitaka. The monastic code (Patimokkha) followed by Thai monks is taken from the Pāli Theravada Canon.

The second major influence on Thai Buddhism is Hindu beliefs received from Cambodia, particularly during the Sukhothai period. Vedic Hinduism played a strong role in the early Thai institution of kingship, just as it did in Cambodia, and exerted influence in the creation of laws and order for Thai society as well as Thai religion. Certain rituals practiced in modern Thailand, either by monks or by Hindu ritual specialists, are either explicitly identified as Hindu in origin, or are easily seen to be derived from Hindu practices. While the visibility of Hinduism in Thai society has been diminished substantially during the Chakri dynasty, Hindu influences, particularly shrines to the god Brahma, continue to be seen in and around Buddhist institutions and ceremonies.

Folk religion—attempts to propitiate and attract the favor of local spirits known as phi—forms the third major influence on Thai Buddhism. While Western observers (as well as urbane and Western-educated Thais) have often drawn a clear line between Thai Buddhism and folk religious practices, this distinction is rarely observed in more rural locales. Spiritual power derived from the observance of Buddhist precepts and rituals is employed in attempting to appease local nature spirits. Many restrictions observed by rural Buddhist monks are derived not from the orthodox Vinaya, but from taboos derived from the practice of folk magic. Astrology, numerology, and the creation of talismans and charms also play a prominent role in Buddhism as practiced by the average Thai—practices that are proscribed in Buddhist texts (see Digha Nikaya 2, ff).

Additionally, more minor influences can be observed stemming from contact with Mahayana Buddhism. Early Buddhism in Thailand is thought to have been derived from an unknown Mahayana tradition. While Mahayana Buddhism was gradually eclipsed in Thailand, certain features of Thai Buddhism—such as the appearance of the bodhisattva Lokesvara in some Thai religious architecture, and the belief that the king of Thailand is a bodhisattva himself—reveal the influence of Mahayana concepts.

The only other bodhisattva prominent in Thai religion is Maitreya, often depicted in Budai form, and often confused with Phra Sangkajai (Thai: พระสังกัจจายน์), a similar but different figure in Thai Buddhist folklore. Images of one or both can be found in many Thai Buddhist temples, and on amulets as well. Thai may pray to be reborn during the time of Maitreya, or dedicate merit from worship activities to that end.

In modern times, additional Mahayana influence has stemmed from the presence of Chinese immigrants in Thai society. While some Chinese have "converted" to Thai-style Theravada Buddhism, many others maintain their own separate temples in the East Asian Mahayana tradition. The growing popularity of the goddess Kuan Yin in Thailand (a form of Avalokitesvara) may be attributed to the Chinese Mahayanist presence in Thailand.

The Happy Buddha...