Prasatthong (Thai: ปราสาททอง) (reigned 1629–1656) was the first king of the Prasat Thong dynasty, the 4th dynasty of the Siamese Ayutthaya kingdom.
Accounts vary on the origin of Prasat Thong. While traditional Thai historians hold that he was an illegitimate son of King Ekathotsarot, Jeremias van Vliet's account states that he was the maternal cousin of King Songtham – his father was Okya Sri Thammathirat (Thai: ออกญาศรีธรรมาธิราช), elder brother of the mother of King Songtham. He was born during the reign of King Naresuan around 1600 and was known to cause mischief in the royal court. He ruined the palace Agricultural Initiation Ceremony, royal ceremony of ploughing, and was threatened with imprisonment; only pleas from the queen of King Naresuan, Chao Khruamanichan, won a reduction of the punishment to five months imprisonment. He was later pardoned and given the title of Okya Sri Vorawong (Thai: ออกญาศรีวรวงศ์), or Phraya Siworawong – a high-ranking title of royal page.
The rise of Prasat Thong to power was documented in van Vliet’s The Historical Account of the war of Succession following the death of King Pra Interajatsia (1650). As the king's maternal cousin, he held great influence. It is said that he was a very ambitious prince and wanted to become a king. King Songtham had had his brother Phra Phanpi Srisin or Phra Srisin (The Siamese chronicles say that Phra Srisin was one of the King Songtham's three sons.) as the Front Palace, technically his successor, but a palace faction including Prasat Thong persuaded the king to give the throne instead to his son Prince Chetthathirat. When King Songtham died in 1628, Chetthathirat ascended the throne and a great purge of the mandarins who had supported Phra Srisin was instigated, including the Samuha Kalahom or Defence Minister. Prasat Thong then replaced him as the Defence Minister with the new title of Okya Suriyawong (Thai: ออกญากลาโหมสุริยวงศ์).
Prasat Thong strived to eliminate his allies-turned-rivals – the Okya Kamhaeng who contested him for the throne and Yamada Nagamasa who objected to the takeover of the throne by Prasat Thong. He quickly condemned Okya Kamhaeng to treason and execution followed. And he sent Yamada Nagamasa to the south as the governor of Ligor, away from Ayutthaya. As soon as the Japanese mandarin left the city, only about a month after his ascension, the child-king was deposed and subsequently executed. Suriyawong or Okya Suriyawong crowned himself as the full-fledged King of Siam.
Prasat Thong had acted as "king-maker" before assuming the throne, by performing the double regicide of King Songtham's sons. Yamada, Okya Seniphimok, heard of the coup at Ayutthaya and rebelled. Prasat Thong had him poisoned and then expelled the remaining Japanese.
As a powerful and decisive leader, he promulgated many criminal laws and sometimes, according to Van Vliet, he even executed prisoners by himself.
Siam was a major trading center attracting Europeans merchants. Prasat Thong was interested in controlling the towns in the southern peninsula, perhaps because of profits from overseas trade. Ayutthaya lost northern subjugated principalities such as Chiangmai.
Under Prasat Thong, Cambodia became subject to Siam again. He then built the capital city using Nakhon Thom as a model and built "places of temporary rest on the way to the footprint of the Buddha".
Upon King Prasat Thong’s death in 1656, Chao Fa Chai, his eldest son, succeeded his father as King Sanpet VI.