Phra Bat Somdet Phra Poramenthramaha Mongkut Phra Chom Klao Chao Yu Hua, or Rama IV, (18 October 1804 – 1 October 1868), is the fourth monarch of Siam (Thailand) under the House of Chakri, ruling from 1851-1868.

He is one of the most revered monarchs of the country.

Outside of Thailand, he is best known as the King in the 1951 play and 1956 film The King and I, based on the 1946 film Anna and the King of Siam – in turn based on the 1944 novel about Anna Leonowens' years at his court, from 1862 to 1867.

During his reign, the pressure of Western expansionism is felt for the first time in Siam.

Mongkut embraces Western innovations and initiates the modernization of Siam, both in technology and culture—earning him the nickname "The Father of Science and Technology" in Siam.

Mongkut is also known for his appointment of his brother, Prince Chutamani, as vice-king.

Prince Chutamani is crowned in 1851 as King Pinklao.

Mongkut himself assures the country that Pinklao should be respected with equal honor as himself.

Mongkut's reign is also the time when the power of the House of Bunnag reaches its zenith and becomes the most powerful noble family of Siam.