In 1351, after consolidating power in the central plains of present-day Thailand, Ramathibodi was proclaimed as the first king of the newly established Ayutthaya Kingdom, which would play a crucial role in Southeast Asian history. Prior to becoming king, Ramathibodi served as a lord of the region known as Phra Ruang or U-thong, where he had substantial influence over local affairs. The strategic location of Ayutthaya, situated along vital trade routes and near the convergence of the Chao Phraya River, provided a favorable environment for the rise of an influential state.

His rule initiated a period of expansion and prosperity, as he worked to unify various city-states and tribes, leading to a stronger centralized government. Underhis leadership, trade flourished, and Ayutthaya became an important commercial hub that attracted merchants from China, India, and the Malay Archipelago. This cultural exchange enriched the kingdom, influencing Thai art, religion, and governance. Additionally, the establishment of the Ayutthaya Kingdom represented an important shift in power dynamics in lower Southeast Asia, shaping future political and social landscapes.

As king, Ramathibodi adopted the title 'Phra Ruang' and is credited with laying down the legal and administrative frameworks that would guide the kingdom for centuries. His contributions included the promotion of Theravada Buddhism, which became a central element of Thai identity, further solidifying his legacy. Ramathibodi's reign set the stage for the continuation of the Ayutthaya kingdom, which would endure for over four centuries until its fall to the Burmese.