stylistic:tone
13th century

During Sukhothai Kingdom in 1275 AD, King Ram Khamhaeng composed his war strategy manual of which also mentioned Muay Thai. Later, King Maha Thammaracha I was trained at young not only essential subjects but also real practice which included Muay Thai for self-defence with bare hands and the art of using such weapons. Muay Thai was also spreadably taught at temples which were the Thai people’s learning centers for teaching following the guideline of King Ram Khamhaeng 's war strategy manual.

In 1431 AD, King Borommarachathirat II, the King of Ayutthaya Kingdom, subjugated Siamese forces, included groups of Siamese boxers to assault the Khmers plundering Angkor Thom also known as Fall of Angkor in 1431. King Borommarachathirat II built inscription of Khun Sri Chaiya Raj Mongkol Thep for the victory monument and Siamese warriors' commemoration of which inscribed list of Siamese warriors and assault record in the inscription after the Angkor had already captured as well as written in A Lost Chronicle of Ayutthaya by Michael Vickery stanza 9–11.

The inscription of Khun Sri Chaiya Raj Mongkol Thep (N.M. 78) 1431 AD Side II reads:-

สมเด็จพระอินทรามหาบรมจักรพรรดิธรรมิกราชเป็นเจ้าให้ขุนศรีไชยราชมงคลเทพเอาจตุรงค์ช้างม้ารี้พลไปโจมจับพระนครพิมายพนมรุ้งเป็นราชเสมาแลราบทาบดังพระมโนสากัลป์แลจึงจะละพระราชเสาวนีย์หาขุนศรีไชยราชมงคลเทพแดฝูงมวยลูกขุนทั้งหลาย เอาช้างม้ารี้พลถอยคืนมา

(Translation): King Borommarachathirat II of Ayutthaya commanded his warrior, Khun Sri Chaiya Raj Mongkol Thep to march four military forces to assault Angkor Thom, Phimai and Phanom Rung to become states under his endless mandate. The King then assigned Khun Sri Chaiya Raj Mongkol Thep, groups of Siamese boxers (Muay), military forces with elephants and horses battalions the retreat procession to Kingdom of Ayutthaya.

From 1455 AD in Ayutthaya period Muay Thai was integrated officially with Siamese Royal courts of Ayutthaya called The King's Guard Department (Thai: กรมทนายเลือก) since the reign of King Borommatrailokkanat enacted the Law of the Military and Provincial Hierarchy 1998 BE (1455 AD) which consisted of two director generals with noble title, Khun Phakdeeasa (Thai: ขุนภักดีอาสา) and Khun Yothaphakdee (Thai: ขุนโยธาภักดี) in the hierarchy. There were countless skilled Siamese boxers in The King's Unarmed Guard Division (Thai: กรมนักมวย), a sub-division of The King's Guard Department, picked from competition by the king to be his private guards and patrol the royal palace as major mission. The boxing competitions occur at peace situation as well as to safeguard the king at war.

There was also the law mentioned of Muay in the reign of King Ekathotsarot (1590–1605), called Miscellaneous Laws (Phra Aiyakan Betset) states:-

117 มาตราหนึ่ง ชนชั้นสองเป็นเอกจิกเอกฉันท์ตีมวยด้วยกันก็ดี แลปล้ำกันก็ดี แลผู้หนึ่งต้องเจ็บปวดก็ดี ขั้นหักถึงแก่มรณภาพก็ดี ท่านว่าหามีโทษมิได้...

(Translation): CLAUSE 117 : ARTICLE I. Second class people who fight by boxing (Muay) or wrestling which then results in injuries or death cannot be penalized...

The well-known Siamese boxers during Ayutthaya period include King Naresuan, King Suriyenthrathibodi and Nai Khanom Tom.