The Siamese state based at Ayutthaya in the valley of the Chao Phraya River grew from the earlier kingdom of Lopburi, which it absorbed, and its rise continued the steady shift southwards of the center of the Tai-speaking peoples Its founder, Ramathibodi I (1314–1369, r. from 1351-1369), was known as Prince U Thong before he ascended to the throne. A native of Chiang Saen (now in Chiang Rai Province) he claimed descent from of the Lao royal lineage of Khun Borom. Scholar Charnvit Kasetsiri hypothesized that U Thong might have actually been born to a Chinese merchant family operating in the area of Phetburi. At least one royal chronicle identifies U Thong as the son of one Chodüksethi, apparently a leader of the Chinese merchant community.[1] Ramathibodi's position was likely secured by political marriage and family ties; he was married to a daughter of the ruling family of Suphanburi, and may have also married into an alliance with the rulers of Lopburi. He was probably chosen as a successor of the king of Lopburi.
In 1350, to escape the threat of an epidemic, King U Thong moved his court south into the rich floodplain of the Chao Phraya. On an island in the river he founded a new capital, which he called Ayutthaya, after Ayodhya in northern India, the city of the hero Rama in the Hindu epic Ramayana. U Thong assumed the royal name of Ramathibodi (1350-69) on March 4, 1351. He appointed both his brother-in-law and son to positions of leadership in Suphanburi and Lopburi, respectively, and established his own capital in the new city of Ayutthaya. Ramathabodi's reign bound together the Khmer rulers of Lopburi, the Tai in the west, and the Chinese and Malaysian merchants who inhabited the coastal areas.
Ramathibodi tried to unify his kingdom. In 1360 he declared Theravada Buddhism the official religion of Ayutthaya and brought members of a sangha, a Buddhist monastic community, from Ceylon to establish new religious orders and spread the faith among his subjects. He also compiled a legal code, based on the Indian Dharmashastra (a Hindu legal text) and Thai custom, which became the basis of royal legislation. Composed in Pali, an Indo-Aryan language closely related to Sanskrit and the language of the Theravada Buddhist scriptures, it had the force of divine injunction. Supplemented by royal decrees, Ramathibodi's legal code remained generally in force until the late nineteenth century.
Ramathibodi's death in 1369 sparked a conflict over succession; initially, his son Ramesuan became ruler of Ayutthaya, but Ramesuan later abdicated in favor of Ramathibodi's brother-in-law, Borommaracha. Some sources indicate that the abdication occurred peacefully, while others indicate that Ramesuan's abdication followed a bloody civil war.