Phra Bat Somdet Phra Poramenthra Maha Mongkut Phra Chom Klao Chao Yu Hua (Thai: พระบาทสมเด็จพระปรเมนทรมหามงกุฎ พระจอมเกล้าเจ้าอยู่หัว), or Rama IV, known in English-speaking countries as King Mongkut, was the fourth monarch of Siam (Thailand) under the House of Chakri, ruling from 1851 to 1868.
During his reign, the pressure of Western expansionism was felt for the first time in Siam. 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 his appointing his brother, Prince Chutamani, as Second King, crowned in 1851 as King Pinklao. Mongkut himself assured the country that Pinklao should be respected with equal honor to himself (as King Naresuan had done with his brother Ekathotsarot in 1583). Mongkut's reign was also the time when the power of the House of Bunnag reached its zenith and became the most powerful noble family of Siam.