Phaya Lithai (Thai: พญาลิไท), also known as Phra Maha Thammaracha I (พระมหาธรรมราชาที่ 1) was a king of the Sukhothai kingdom, reigning from roughly 1346 until his death in 1374. Lithai was the son of Lerthai, also known as Loethai or Lelithai. The exact chronology of Lithai's rise to the throne is unclear. Popular tradition names him as the fourth king of Sukhothai, but dynastic records seem to indicate that at least one other king (Nguanamthom) ruled between Lithai and his father, as well as the regent Saisongkhram who ruled during Lerthai's absence following the death of Ram Khamhaeng the Great.
Lithai served as uparat during his father's reign from the city of Srisatchanalai, an important urban center of the early Sukhothai kingdom.
Lithai is known as the writer of the Traiphuum Phra Ruang ('Three worlds of Phra Ruang', Phra Ruang being the dynastic name of Lithai's linneage), a religious text describing the various world of Buddhist cosmology, and the way in which karma consigns living beings to one world or another. The Traiphuum would go on to serve as an important political document, being re-interpreted in response to changes in the domestic and international political scene. King Lithai also built the biggest Buddha image in the northern territory of Siam called Phraputtachinnarat.