King Athittayawong (Thai: อาทิตยวงศ) was the shortest-reigning monarch of Ayutthaya, only for 36 days in 1629 and the last king of the Sukhothai dynasty.
Prince Athitayawong was the son of King Songtham and his principal wife. His elder brother, King Chetthathirat and his mother were executed in 1629 after Okya Siworawong (Thai: ออกญาศรีวรวงศ์) or Phraya Siworawong staged a coup.
The ten years old child prince was raised to the Ayutthaya throne with Siworawong. However, Siworawong continued to administer the country as Chaophraya Kalahom. After about a month, the government servants complained that there were in fact two chiefs in the country, Athittayawong and Siworawong administering as a king, which could cause future danger for the country. They persuaded Siworawong to ascend to the throne so there would be only one king as per custom. Athittayawong was dethroned and Siworawong assumed the title Somdet Phrachao Prasat Thong.
Athitayawong was executed at Wat Kok Phraya (Thai: วัดโคกพระยา) soon, ending the Sukhothai dynasty. Siworawong ascended the throne as the first king of the Prasat Thong dynasty.