U-thong (Thai: อู่ทอง) or Ramathibodi I (Thai: รามาธิบดีที่ 1) (1314–1369) was the first king of the kingdom Ayutthaya (now part of Thailand), reigning from 1350 to 1369. He was known as Prince U Thong (meaning "Golden Cradle") before he ascended to the throne in 1350. He was thought to have been a descendant of Mangrai.

Ramathibodi I established four Great Officers of State. These were the Ministry of the Interior (Wieng), the Ministry of the Treasury (Klang), the Ministry for the King's Household (Wang), and the Ministry of Agriculture (Na). He also codified the T'ai laws. Finally, he formed an alliance with the Ming Dynasty.

He established his own capital in the new city of Ayutthaya. King Ramathabodi's reign included Korat, Chantaburi, Tavoy, Tenasserim, and large parts of Malaya.

According to a better-known source, a seventeenth-century account by Dutchman Jeremias Van Vliet, a Renowned Legend stated that Ramatibodi was an ethnic Chinese, having sailed down from China. After succeeding in trade, he became influential enough to rule the city of Phetchaburi, a coastal town of the Gulf of Thailand, before travelling up to Ayutthaya.

In 1352 he laid siege to Yasodharapura. He was successful the following year and placed one of his sons on the throne. However, they were only able to keep the throne until 1357, when the Khmers were able to regain it.

King Ramathibodi's death sparked a conflict over succession. Initially, his son King Ramesuan became ruler of Ayutthaya, but King Ramesuan later abdicated in favor of King Ramathibodi's brother-in-law, King Borommaracha I. This rivalry for the throne may have been peaceful or it may have been bloody.

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