This article examines the legal basis underpinning the application of international humanitarian law treaties to non-state armed groups. Although it is widely accepted that international humanitarian law does bind armed groups, the legal basis remains uncertain and is happily—if somewhat disbelievingly—accepted. Yet, the importance of understanding the legal basis underpinning this attribution is evident, not only in terms of legal clarity and principles such as nullen crimen sine lege, but also, and perhaps more significantly, as a means of facilitating the future regulation of non-state armed groups—and indeed other non-state actors—by means of international treaty law. The customary law, third-party consent and legislative/prescriptive jurisdiction theories are addressed in detail. Significantly, the third-party consent theory is rejected on the basis of the non-applicability of the pacta tertiis principle to non-state actors consequent to such entities distinguished international legal personality. Rejection of the third-party consent theory overcomes a key obstacle vis-à-vis the application of international treaty law to non-state actors, removing any possible legal requirement that armed groups consent to obligations arising under international humanitarian law. The legislative jurisdiction theory is then discussed and accepted as a coherent basis for the direct application of international treaty law to non-state groups, establishing that non-state armed groups may be bound by the treaty obligations of the territorial state. Importantly, this principle may be used as a basis for the future attribution of international treaty law to non-state actors, for example, in the field of human rights.