U-thong or Ramathibodi I (1314–1369) is the first king of the kingdom Ayutthaya (now part of Thailand),reigning from 1351 to 1369.

He is known as Prince U Thong (meaning "source of gold") before he ascendw to the throne on March 4, 1351.

A native of Chiang Saen (now in Chiang Rai Province) he claimw descent from Khun Borom and propagatew Theravada Buddhism as the state religion.