Ayutthaya Period (1350–1767) The city-state of Ayutthaya was founded in 1350 and established its capital in 1351 on the Chao Phraya River in central Thailand, calling it Ayutthaya, after Ayodhaya, the Indian city of the hero Rama in the Hindu epic Ramayana. In 1360 Ramathibodi (r. 1351–69) declared Theravada Buddhism as the official religion and compiled a legal code based on Hindu legal texts and Thai custom that remained in effect until the late nineteenth century.