Phraya Phahonphonphayuhasena (29 March 1887 – 14 February 1947), simply known as Phraya Phahon, was a Thai military leader and politician. Born Phot Phahonyothin and briefly educated in Germany, he was a member of the Khana Ratsadon and launched a coup d'état to become Prime Minister of Siam in 1933, a position he held until 1938.

Phot was born in Phra Nakhon Province (present-day Bangkok) to a Teochew Thai Chinese father, Colonel Kim (กิ่ม), and a Thai Mon mother, Chap (จับ). His family took the surname Phahonyothin in 1913 per royal decree.

After attending the Chulachomklao Royal Military Academy, in 1903 Phot was sent by royal scholarship to study at the Prussian Military Academy (Preußische Hauptkadettenanstalt) in the town of Lichterfelde, near Berlin, German Empire, where he was allegedly a classmate of Hermann Göring.

He was then sent to study at the Engineering College of Copenhagen in Denmark, but was only able to complete one year as his scholarship funds ran out, forcing him to return to Siam in 1912. In 1931 he was elevated to the title of Phraya Phahonphonphayuhasena by King Prajadhipok (or Rama VII) and received the rank of colonel. In 1932 he became Commander of the Royal Siamese Army.

Phraya Phahon was an important member of a group of conspirators known as the "Four Musketeers" (4 ทหารเสือ). They were part of the Khana Ratsadon (or 'People's Party') who carried out the revolution of 1932. After the 1932 coup, three factions formed among the political and military leaders of Khana Ratsadon: the senior military faction led by Phraya Phahon; the junior army and navy faction led by Luang Phibunsongkhram; and the civilian faction led by Pridi Banomyong. Also he, Phraya Songsuradet, and Phraya Rithi-akkhaney were the collective military defenders of capital and exercised power on behalf of the king.

As its most senior member, Phraya Phahon was viewed as the de facto leader of the Khana Ratsadon and the revolution itself. It was Phraya Phahon who read the Declaration of the New Siamese State in the Royal Plaza that declared the end of absolute monarchy and the establishment of the constitutional Siamese state. Because of the key role he played in the revolution he was rewarded with a high position in the new government and was made a member of the new cabinet.

In March 1933 a constitutional crisis developed in Siam over the "Yellow Cover Dossier" incident, precipitated by Pridi's draft economic plan, which contained socialist elements. Because of this, Phraya Manopakorn Nititada, the prime minister, expelled him from the cabinet and suspended the constitution. This action upset many in the People's Party who supported Pridi, including Phraya Phahon. On 15 June Phraya Phahon resigned from the cabinet citing health reasons. In truth, he and a couple of military officers planned to overthrow Phraya Mano's increasingly authoritarian government. On 20 June a bloodless coup was carried out, led by Phraya Phahon. The following day, Phraya Phahon appointed himself the second Prime Minister of Siam and immediately sent a report to King Prajadhipok explaining the objectives of the coup. The king reluctantly endorsed him and exiled Phraya Mano to British Malaya.

The next five years were a struggle to maintain power. In October 1933, a royalist revolt against Phraya Phahon's government, the Boworadet Rebellion, occurred only four months after his becoming prime minister. After weeks of fighting, government troops emerged victorious and Phraya Phahon was able to solidify his position. The cabinet was divided politically. The government was maintained only by the force of his personality. Luang Phibunsongkhram, a trusted confidant, became minister of defence under the new government and he began to acquire greater power and influence in preparation for his eventual ascension to the premiership.