The earliest records of Kun Khmer can be traced all the way back to the Khmer Empire which ruled from the 9th to the 15th centuries. Initially, it was developed by the Khmer military as a hand-to-hand combat system to fight against armed and unarmed enemies. During this period, the empire ruled most of the modern Southeast Asian countries like Thailand, Cambodia, and Vietnam, with most of these countries adopting the practice of Kun Khmer.

France organized its own athletes to face off against Cambodian athletes. Recognizing the impressive capabilities of Khmer martial arts, particularly in the use of elbows, knees, fists, and other lethal techniques, including those that could result in neck injuries, the French administration introduced a modified version of boxing. This new sport mandated that all participants wear gloves, imposed time limits on matches, instituted rest periods, and prohibited certain techniques to mitigate risks to life. The intention behind this French initiative was to restrict the abilities of Khmer competitors while providing an opportunity for French athletes familiar with Khmer martial arts to secure victories in the contests. Subsequently, this definition evolved into the established rules of the game, leading to its recognition as a popular sport. Freestyle boxing (pradal serey) represents a fusion of the universal sport of boxing, rooted in European tradition, and the traditional martial arts of the Khmer culture. Through an agreement between French and Cambodian working groups, specific regulations were implemented, including prohibitions on additional time for falls, neck strikes, the use of protective gear, and other lethal techniques. This sport gained popularity during the French colonial period and subsequently spread to Thailand, Vietnam, Laos, and various other nations under the French sphere of influence. 

The introduction of boxing rings, rounds, and gloves, as well as the prohibition of life-threatening techniques, transformed the local martial arts into a sport. With those new sets of rules, French and Khmer boxers would compete against each other in tournaments.[16] In the 1960s, Cambodian boxing promoters held inter-martial arts exhibitions.