Siam is renamed Thailand by Plaek Phibunsongkhram, the country's third prime minister.

Plaek Phibunsongkhram 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.

The prime minister of Thailand is the head of government of Thailand. The prime minister is also the chair of the cabinet of Thailand. The post has existed since the Siamese Revolution of 1932, when the country became a constitutional monarchy. Prior to the 2014 coup d'état, the prime minister was nominated by a vote in the Thai House of Representatives by a simple majority, and is then appointed and sworn in by the King of Thailand. The house's selection is usually based on the fact that either the prime minister is the leader of the largest political party in the lower house or the leader of the largest coalition of parties. In accordance with the 2017 Constitution, the Prime Minister can hold the office for no longer than eight years, consecutively or not. Paetongtarn Shinawatra was appointed Prime Minister of Thailand on 16 August 2024 following the Constitutional Court's ruling that terminated Srettha Thavisin’s premiership due to the unlawful appointment of an unqualified minister, Phichit Chuenban. In July 2025, her own premiership was challenged after an audio recording of her conversation with Cambodian Prime Minister Hun Sen regarding a border closure was leaked, prompting a petition to the Constitutional Court. On 1 July 2025, the court ordered her to suspend duties pending a decision. During the interim, Deputy Prime Minister Suriya Juangroongruangkit served as acting Prime Minister, as the first-ranking Deputy Prime Minister, Phumtham Wechayachai, had not yet received formal royal endorsement. Upon his swearing-in on 3 July 2025, Phumtham assumed the role of acting Prime Minister.