Prasatthong (Thai: ปราสาททอง) (c.1600-1656) (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).

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