King Mengrai (or Mangrai) (1239-1317) was the founder of the Thai kingdom Lannathai.

Mengrai was born in Chiang Saen as son of the local ruler Lao Meng and Princess Ua Ming Chommuong , from the ruling family of the Thai Lue of Chiang Rung, Yunnan. 1259 Mengrai succeeded his father. In 1262 he founded the city Chiang Rai as the new capital. The kingdom grew quickly to the south by both alliances and also conquests of neighbors. In 1281 (or 1292) he defeated the Mon kingdom of Haripunchai, adding Lamphun to his kingdom. In 1287 Mengrai made a peace pact with King Ramkhamhaeng the Great of Sukhothai and Ngam Muang, ruler of the smaller principality of Phayao. In 1296 Mengrai moved his capital south by founding Chiang Mai (new city).

Mengrai died in 1317 in Chiang Mai. During a thunderstorm he was hit by lightning on the market of the city. He was succeeded by his grandson Saen Phu, but the throne was soon thereafter usurpated by Khun Krua, but could regain the thron in 1324.