The emergence of Buddhism in the region that is now Thailand unfolded gradually, through several overlapping currents of influence rather than a single decisive moment. Traditions associated with Emperor Aśoka speak of Theravāda missions to Suvarṇabhūmi in the 3rd century BCE, suggesting an early legendary horizon in which the Dhamma was carried across the Bay of Bengal. More concrete indications appear later, when archaeological evidence points to Buddhist presence from around the early centuries of the Common Era, facilitated by trade and cultural exchange with the Indian subcontinent. These early contacts prepared the ground for a more stable and visible Buddhist culture to take root.

A clearer historical profile emerges with the rise of Mon polities such as Dvāravatī in central regions of present‑day Thailand from about the 5th–6th centuries CE. In these kingdoms, Theravāda Buddhism became firmly established, as reflected in inscriptions, Buddha images, and stupas that testify to a living monastic and lay community. During this period, Theravāda lineages linked to Sri Lanka played a significant role in shaping doctrine and practice, and Mon territories became important conduits through which the Thai-speaking peoples later encountered the Buddhist tradition. The Khmer cultural sphere also contributed Buddhist influences alongside Hindu elements, further enriching the religious landscape into which Thai polities would eventually step.

As Thai principalities gained strength, especially by the 13th century, Buddhism was not merely adopted but consciously re‑articulated as a foundation for political and social order. In the Sukhothai kingdom, rulers actively sought religious prestige and orthodoxy from Sri Lanka, inviting monks and texts that helped to standardize Theravāda teaching and monastic discipline. This reform and consolidation gave Thai Buddhism its characteristic emphasis on the Pali Canon, the centrality of the sangha, and the cultivation of merit through generous and disciplined conduct. Over time, these currents converged so that Buddhism, once a foreign teaching carried by merchants and missionaries, became the spiritual heart of Thai society and a primary source of legitimacy for its kings.