Borommatrailokkanat (Thai: บรมไตรโลกนาถ, pronounced [bɔ̄ː.rōm.mā.trāj.lôːk.kā.nâːt], Sanskrit: Brahmatrailokanātha) 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 Buddhist belief, was a "glorious and happy sign". His reign was also known for massive reforms of Thai bureaucracy and a successful campaign against the Lan Na Kingdom to the north. He was revered as one of the greatest monarchs of Thailand.

Ramesuan (not to be confused with King Ramesuan r. 1369–1370) was born in 1431 to King Borommarachathirat II or Chao Sam Phraya. Some authors claim that his mother was a princess of the Sukhothai Kingdom, daughter of Sai Lue Thai. According to historian Michael Vickery, however, this is not verifiable in historic sources and may be due to a misreading or misinterpretation of a chronicle.

Be that as it may, Ramesuan was born at a time when the Kingdom of Sukhothai was increasingly tied to Ayutthaya, being under its suzerainty since 1412. When King Maha Thammaracha IV (Borommapan) of Sukhothai died in 1438, Borommaracha II of Ayutthaya annexed Sukhothai and installed his own son, then seven year old Prince Ramesuan, as viceroy (uparaja) of Sukhothai. When Ramesuan was 15 years old, the king sent him to Phitsanulok (which had replaced the city of Sukhothai as capital around 1430) to rule the former territories of Sukhothai which were now known as the "northern cities" (Mueang Nuea) and increase the control of the ruling family over them.

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.

The traditional ministries of Thailand—the Kalahom (predecessor of today's ministry of defence) and the Mahatthai (precursor of the ministry of interior)—were introduced by Borommatrailokkanat. Originally they were meant to serve as simultaneous military and civil Greater Officers of State for the north and south, respectively.

Also in his reign in 1454, noble titles were first codified under the hierarchy system called sakdina. Each subject was given a numerical ranking corresponding to the number of rai of land he was entitled to.

Borommatrailokkanat adopted the position of Uparaja, translated as "Deputy-King", "viceroy" or "underking", usually held by the king's senior son, or full brother, or the sons of their queens, in an attempt to regularize the succession to the throne — a particularly difficult feat for a polygamous dynasty. In practice, there was inherent conflict between king and uparaja and frequent disputed successions. He appointed his eldest son Uparaja of Ayutthaya while he lived in Phitsanulok for twenty-five years, keeping Chiang Mai under control. Eventually, a younger son was made the Uparaja of Phitsanulok, junior to the Uparaja of Ayutthaya.