Plaek Phibunsongkhram (14 July 1897 – 11 June 1964) was a Thai military officer and politician who served as the third prime minister of Thailand from 1938 to 1944 and again from 1948 to 1957. He rose to power as a leading member of the Khana Ratsadon, becoming prime minister in 1938 and later consolidating his influence as a military dictator. His regime allied with the Empire of Japan during World War II, and his administration was marked by authoritarian policies and the promotion of Thai nationalism. He was closely involved in both domestic reforms and foreign policy during the war and played a central role in shaping modern Thai state ideology.

Phibun was a member of the army wing of Khana Ratsadon, the first political party in Thailand, and a leader of the Siamese revolution of 1932, which replaced Thailand's absolute monarchy with a constitutional monarchy. Phibun became the third Prime Minister of Thailand in 1938 while serving as Commander of the Royal Siamese Army. Inspired by the Italian fascism of Benito Mussolini, he established a de facto military dictatorship run along fascist lines, promoted Thai nationalism and Sinophobia, and allied Thailand with Imperial Japan in World War II. Phibun launched a modernization campaign known as the Thai Cultural Revolution that included a series of cultural mandates, which changed the country's name from "Siam" to "Thailand", and promoted the Thai language.

Phibun was ousted as prime minister by the National Assembly in 1944 and replaced by members of the Free Thai Movement, but returned to power after the Siamese coup d'état of 1947, led by the Coup Group. Phibun aligned Thailand with anti-communism in the Cold War, entered the Korean War under the United Nations Command, and abandoned fascism for a façade of democracy. Phibun's second term as prime minister was plagued by political instability and several attempts to launch a coup d'etat against him were made, including the Army General Staff plot in 1948, the Palace Rebellion in 1949, and the Manhattan Rebellion in 1951. Phibun attempted to transform Thailand into an electoral democracy from the mid-1950s onward, but was overthrown in 1957 and went into exile in Japan, where he died in 1964.

At fifteen years and one month, Phibun's term as Prime Minister of Thailand was the longest to date.

Phibun was born Plaek on 14 July 1897 in Mueang Nonthaburi, Nonthaburi Province, in the Kingdom of Siam to durian farmers. His family began using the surname Khittasangkha (Thai: ขีตตะสังคะ) after a 1913 decree on surnames.

He received his given name – meaning "strange" or "weird" in English – because of his unusual appearance as a child where his ears were positioned below his eyes, rather than above his eyes like others.

Plaek's paternal grandfather was a Chinese immigrant from Guangdong of Cantonese descent. However, the family was completely assimilated, being considered Central Thai people, since most of the Chinese in Thailand are from the Teochew dialect group, Plaek did not pass the criteria for being considered Chinese as well, enabling him to successfully conceal and deny his Chinese roots.

He studied in Buddhist temples before joining the Royal Military Academy; upon graduation in 1914, he was commissioned into the Royal Siamese Army as a second lieutenant in the artillery. Following World War I, he was sent to France to study artillery tactics at the École d'application d'artillerie. In 1928, as he rose in rank, he received the noble title Luang from King Prajadhipok, and became known as Luang Phibunsongkhram. He would later drop his Luang title but permanently adopted Phibunsongkhram as his surname.