king of Siam (1851–68) who opened his country to Western influence and initiated reforms and modern development.

Mongkut was the 43rd child of King Rama II, but as the first son to be born of a queen he was favoured to succeed to the throne. When his father died in 1824, however, Mongkut was barely 20, and the royal accession council instead chose his older and more experienced half brother to reign as King Phranangklao (Rama III). To hold aloof from politics, Mongkut chose to become a Buddhist monk. A few years later he encountered a particularly pious monk who inspired Mongkut to turn to the strict discipline and teachings of early Buddhism. He became an accomplished scholar and abbot of a Bangkok monastery, which he made a centre of intellectual discourse that gradually came to involve American and French Christian missionaries and the study of Western languages and science. Mongkut also was able to travel in the countryside as no previous Thai king had done. The reformed Buddhism that Mongkut developed gradually grew into the Thammayut order, which to the present day is at the intellectual centre of Thai Buddhism. Mongkut's friends in the 1840s included many leading princes and nobles who similarly were excited by the West. Convinced of the necessity of accommodation with the West, they took the lead in managing the succession of Mongkut to the throne when King Rama III died in 1851. The leader of that group, Somdet Chao Phraya Si Suriyawong (Si Suriyawong, Somdet Chao Phraya), became Mongkut's effective prime minister, and together the two successfully concluded treaties with Great Britain, the United States, and other powers beginning in 1855 that fully opened Siam to Western commerce. Thai concessions staved off Western imperial pressure for another generation and brought rapid economic development, but Siam had to concede extraterritoriality and limits on her taxing and tariff policies. To win recognition as an equal among the world's rulers, Mongkut corresponded with them, even offering to send elephants to U.S. Pres. James Buchanan to assist in the development of the United States. His shrewd foreign policy balanced Britain and France against each other to ensure Siam's survival. His tolerance and open-mindedness proved far more effective in dealing with Western imperialists than the xenophobia and isolationism of some of his neighbouring rulers. For a time the royal household employed an English governess, Anna Leonowens (Leonowens, Anna Harriette) (q.v.), whose published reminiscences made Mongkut the model for the king in a 20th-century musical comedy, The King and I.

In his own reign Mongkut was unable to achieve fundamental internal reforms, but he took pains to ensure the liberal education of his sons, who in the next generation would begin the modernization of Siam.