King Borommakot^[3] (Thai: สมเด็จพระเจ้าอยู่หัวบรมโกศ, pronounced [bɔ̄ː.rōm.mā.kòːt]) or King Maha Thammarachathirat II (Thai: สมเด็จพระมหาธรรมราชาธิราชที่ ๒) was the king of Ayutthaya from 1733 to 1758. His reign was the last blooming period of Ayutthaya as the kingdom would fall nine years after his death.^{[4]: 68–69}

"His reign of 25 years is important for being the last peaceful period of Ayudhya during which literature with the arts and crafts flurished." However, the king himself was known for "cruelty to people and animals alike," with seven of his sons meeting violent deaths.^{[4]: 67–68}

Much of what survives in Ayutthaya today dates back to Borommakot's massive renovations of Ayutthaya temples in the second quarter of the 18th century.^[5] King Rama I attempted to emulate the religious customs of Ayutthaya during Borommakot's reign in the early Bangkok period and even postponed his coronation until he was certain that his coronation was confidently modelled off of Borommakot's coronation.^[6]

Prince Phon (Thai: พร) was the son of Sanpet VIII. His elder brother, Prince Phet (เพชร), succeeded the throne as Sanphet IX (Thai Sa) in 1708.^[7]: 277 Phon was then appointed as the Front Palace. However, Thai Sa decided that upon his death the throne would be given to his second eldest son, Prince Aphai, since Thai Sa's eldest son had entered the priesthood. In 1732, Thai Sa died and a civil war commenced. Phon led his armies against his nephews, Prince Aphai and Prince Paramet. The civil war within Ayutthaya was "a big fight, bigger than any which had occurred in Siam in former times". With the victory ensured, Phon executed his nephews, the government civil servant allies, and took the throne as King Borommakot.^{[7]: 280–281}

Because the Samuha Kalahom had lent the support to Prince Aphai, Borommakot removed the power of Samuha Kalahom by depriving its authorities over southern Siam and transferred the power to Kromma Tha instead. The Samuha Kalahom remained as a mere military figurehead.

In spite of the bloodshed that preceded his reign, Borommakot was known for his reconstruction of Buddhist temples and the peace and prosperity Ayutthaya finally enjoyed again. In 1753, Borommakot sent two Siamese monks to rehabilitate Theravada Buddhism in Sri Lanka.^{[7]: 282–283, 295}

In 1741, Borommakot made his son Thammathibet the Grand Vizier. Thammathibet proved to be an able prince and was also known for being a poet. However, Thammathibet had affairs with two of Borommakot's concubines, Princess Sangwan^[7]: 296 and Princess Nim—a severe crime. The lovers were caught in 1746 and the three were beaten. The Grand Vizier was lashed 120 strokes and he died while he was beaten, and the two concubine princesses 30 lashes each. Princess Sangwan died 3 days later. Princess Nim survived, but she was banished from the court.

Borommakot then appointed his third son, Duea (เดื่อ; later became Uthumphon), as the Grand Vizier. Borommakot skipped his second son, Ekkathat, because he thought that Ekkathat was not suitable to be a king.^[7]: 297

However, Ekkathat still had designs on the throne, and the subsequent struggles of the princes for the throne would contribute to the fall of Ayutthaya in 1767, during the reign of Ekkathat.^{[7]: 298–300}

Boromakot was revered by the later Bangkok aristocracy for his religious reforms at attempting to solve the issues of social turmoil, something which the later Bangkok aristocracy would attempt to implement in the Rattanakosin Kingdom under King Rama I's reign.

Rama I attempted to emulate the religious customs of Ayutthaya during Borommakot's reign in the early Bangkok period and even postponed his coronation until he was certain that his coronation was confidently modelled off of Borommakot's coronation.^[9]