For centuries, Buddhism was said to be the main source of law in Buddhist kingdoms in mainland Southeast Asia, as seen in the prevalence of the cult of Dhammasastra. But what is Buddhist about these Buddhist laws? Buddha had never taught a legal code, only dhamma and vinya for monastic life, so the relationship between Buddhism and law is not straightforward. Dhamma itself is cosmic law. Vinya is a monastic code of conduct. This chapter aims to explore how the three types of ‘law’, dhamma, vinya, and Buddhist law, related with one another in ancient Siam. How did Buddhism shape legal, and political, arrangement in Siam? More importantly, when the service of Buddhist law officially came to an end in the early 20th Century, is there any residue of the idea left in the modern legal thought and culture of today Thailand?