Chulalongkorn (จุฬาลงกรณ์, 20 September 1853 – 23 October 1910) was the fifth monarch of Siam under the House of Chakri, titled Rama V from 1 October 1868 to his death in 23 October 1910.
Chulalongkorn was born as the son of King Mongkut in 1853. In 1868, he travelled with his father and other Westerners invited by Mongkut to observe the solar eclipse of August 18, 1868 in Prachuap Khiri Khan Province. However, they both contracted malaria which resulted in his father's death.
Chulalongkorn's reign was characterised by the modernisation of Siam, governmental and social reforms, and territorial concessions to the British and French. As Siam was surrounded by European colonies, Chulalongkorn, through his policies and acts, ensured the independence of Siam. All his reforms were dedicated to ensuring Siam's independence given the increasing encroachment of Western powers, so that Chulalongkorn earned the epithet Phra Piya Maharat (พระปิยมหาราช, the Great Beloved King).
King Chulalongkorn was born on 20 September 1853 to King Mongkut and Queen Debsirindra and given the name Chulalongkorn. In 1861, he was designated Krommamuen Pikhanesuan Surasangkat. His father gave him a broad education, including instruction from Western tutors such as Anna Leonowens. Chulalongkorn, along with his siblings, were educated by Leonowens from her arrival in August 1862 through to her departure in 1867. During this time, Chulalongkorn became friends with Leonowens' son Louis Leonowens who was 2 years younger then Chulalongkorn. His friendship with Louis would continue his adulthood where he assisted Louis' business in Siam.
In 1866, he became a novice monk for six months at Wat Bawonniwet according to royal tradition. Upon his return to his secular life in 1867, he was designated Krommakhun Phinit Prachanat (กรมขุนพินิตประชานาถ.)
In 1867, King Mongkut led an expedition to the Malay Peninsula south of the city of Hua Hin, to verify his calculations of the solar eclipse of 18 August 1868. Both Mongkut and his son fell ill of malaria.