Chettathirat (Thai: เชษฐาธิราช) or (upon accession to the Ayutthayan throne) Ramathibodi II (Thai: รามาธิบดีที่ 2) was the King of Sukhothai from 1485 and King of Ayutthaya from 1491 to 1529. His reign was marked by the first Western Contact with the Portuguese.
Prince Chettathirat was the youngest of Trailokanat's three sons. His eldest brother, Prince Borommaracha, was appointed the regent of Ayutthaya during his father's campaigns against Lanna kingdom. His another brother, Prince Indraracha, died during the wars with Lanna. In 1485, Prince Chettathirat was appointed the Uparaja, or Crown Prince, and was crowned as the King of Sukhothai (The title King of Sukhothai was the title of Ayutthayan Crown Prince.)
In 1488, Trailokanat died. Though Chettathirat was the Crown Prince, the Ayutthayan throne was inherited by his brother Prince Borommaracha, as Borommaracha III. In 1491, Borommaracha III died, leaving the throne to Chettathirat, thus reuniting the two kingdoms.
Chettathiraj took the reigning name in Ayutthaya as Ramathibodi II.
Ramathibodi II died in 1529, during which a great comet appeared as recorded in the Siamese Chronicles. His son, Prince Athittayawong, succeeded to the throne as Borommaracha IV.