Borommatrailokkanat (Thai: บรมไตรโลกนาถ) or Trailok (1431–1488) was the king of the Ayutthaya Kingdom from 1448 to 1488. He was one of many monarchs who gained the epithet King of White Elephants (Thai: พระเจ้าช้างเผือก). He was the first Thai king to possess a "noble" or white elephant, which, according to Hindu belief, was a "glorious and happy sign". His reign was also known for a massive reforms of Thai bureaucracy and a successful campaign against Lan Na. He was also revered as one of the greatest monarchs of Thailand.

Prince Ramesuan (not to be confused with King Ramesuan r. 1369–1370) was born in 1431 to King Borommarachathirat II or Chao Sam Phraya and his queen from the Sukhothai Kingdom. He became the Uparaja (lit. vice-king of crown princes) in 1438. When his cousin, Borommapan of Sukhothai, died in 1438, Ramesuan was then technically the king of Sukhothai although he was too young to be crowned. Upon reaching majority, Borommaracha II sent Ramesuan to Phitsanulok to assume the Sukhothai throne.

Borommaracha II died in 1448, Prince Ramesuan was then crowned King Borommatrailokkanat of Ayutthaya, making a personal union between Sukhothai and Ayutthaya.

Borommatrailokkanat reformed the Thai bureaucracy – the system lasted well into the 20th century. He separated civil and military officials, giving them titular ranks and feudal ranks to create the hierarchy of nobility, or life-nobles. He also established the mandalas: Inner Cities, Outer Cities, and Tributaries. He also stopped the tradition of appointing royal princes to govern cities, as they had always clashed with each other in times of succession. Borommatrailokkanat promulgated Ayutthayan Law in 1458.

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