Mongkut (มงกุฏ; 18 October 1804 1 October 1868) was the fourth monarch of Siam (Thailand) under the House of Chakri, titled Rama IV. He ruled from 1851 to 1868. His full title in Thai was Phra Bat Somdet Phra Menthora Ramathibodi Sri Sinthara Mahamakut Phra Mongkut Phra Siam Deva Mahamakut Wittaya Maharaj (พระบาทสมเด็จพระปรเมนทรรามาธิบดีศรีสินทรมหามงกุฎ พระจอมเกล้าเจ้าอยู่หัว พระสยามเทวมหามกุฏวิทยมหาราช).
Outside Thailand, Mongkut is best known as the king in the 1951 musical and 1956 film The King and I, based on the 1946 film Anna and the King of Siamin turn based on a 1944 novel by an American author about Anna Leonowens' years at his court, from 1862 to 1867, drawn from Leonowens’ memoir.
Siam first felt the pressure of Western expansionism during Mongkut's reign. Mongkut embraced Western innovations and initiated the modernization of his country, both in technology and culture—earning him the nickname "The Father of Science and Technology" in Siam.
Mongkut was also known for appointing his younger brother, Prince Chutamani, as Second King, crowned in 1851 as King Pinklao. Mongkut told the country that Pinklao should be respected with equal honor to himself (as King Naresuan had done with his brother Ekathotsarot in 1583). During Mongkut's reign, the power of the House of Bunnag reached its zenith: It became the most powerful noble family of Siam.
Mongkut (มงกุฎ, literal meaning: crown) was the second son of Prince Isarasundhorn, son of Phutthayotfa Chulalok, the first Chakri king of Siam (King Rama I) and Princess Bunreod. Mongkut was born in the Old (Thonburi) Palace in 1804, where the first son had died shortly after birth in 1801. He was followed by Prince Chutamani (เจ้าฟ้าจุฑามณี) in 1808. In 1809, Prince Isarasundhorn was crowned as Buddha Loetla Nabhalai (later styled King Rama II.) The royal family then moved to the Grand Palace. Thenceforth, until their own accessions as kings, the brothers (เจ้าฟ้า chaofa) were called Chao Fa Yai (เจ้าฟ้าใหญ่) and Chao Fa Noi (เจ้าฟ้าน้อย).