Mongkut (Thai: มงกุฏ; 18 October 1804 – 1 October 1868) was the fourth monarch of Siam (Thailand) under the House of Chakri, ruling from 1851 to 1868. His full title in Thai is Phra Bat Somdet Phra Menthora Ramathibodi Sri Sinthara Mahamakut Phra Mongkut Phra Siam Deva Mahamakut Wittaya Maharaj (พระบาทสมเด็จพระปรเมนทรรามาธิบดีศรีสินทรมหามงกุฎ พระจอมเกล้าเจ้าอยู่หัว พระสยามเทวมหามกุฏวิทยมหาราช).

Outside Thailand, he 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 Siam—in turn based on a 1944 novel by an American missionary about Anna Leonowens' years at his court, from 1862 to 1867.Siam first felt the pressure of Western expansionism during his 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 assured 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.