Muay Thai as the traditional martial art of the Thai people. In the last few decades, Muay Thai has spread throughout the world. The International Federation of Muay Thai Amateurs (IFMA), based in Bangkok, boasts 130 member countries. One of the places Muay Thai spread to and became popularized is California, USA. This paper is an ethnographic study of the phenomenon of how the Thai martial art of Muay Thai has been popularized and localized in California. I did field work at Muay Thai gyms in California and Thailand where I observed the different traditions, practices, and training methodologies. I analyzed Google Trends search data and statistics from the US Muay Thai Federation (USMF) and International Kickboxing Federation (IKF) to determine the popularity of Muay Thai, MMA and other martial arts in California. The results of this study show the popularity of Muay Thai in California started with the beginning of the Ultimate Fighting Championship (UFC) in 1993. The UFC featured several different martial arts styles, including Muay Thai, and led to the phenomenon of Mixed Martial Arts (MMA). As Muay Thai gained popularity in California it was localized by entrepreneurs to adapt to the tastes of Californians. The main aspects of traditional Muay Thai that were localized in California are: competition rules, the lack of gambling, the deletion of the Wai Kru, pedagogical methods, kids Muay Thai classes, the band system of promotions, Muay Thai for fitness, and the teaching of other martial arts at Muay Thai gyms. Interest in Muay Thai has exploded internationally. The inclusion of Muay Thai as a demonstration sport in the 2020 Olympics is sure to increase attention to the sport. Recommendations for further study are to analyze these trends in popularity, as well as international promotions offering Muay Thai fights, such as One Championship, to ensure what they are calling Muay Thai holds true to the image that Thailand wants to promote for the sport. The internationalization of Muay Thai should be carefully studied by stakeholders in Thailand to ensure the country benefits from the popularity of their traditional sport, and the cultural tradition of Muay Thai is preserved.