The implementation of international human right treaties is particularly challenging, especially when they entail obligations that apply at the subnational level. In this article, we examine how international law intermediaries translate and use international treaties in subnational policymaking processes. We develop a dedicated analytical framework, and we derive a typology, characterizing different types of intermediaries and systematizing the ways political-administrative actors use international law at the subnational level. On this basis, our empirical analysis shows how the implementation of a crucial treaty—the Istanbul Convention (IC) on preventing and combating violence against women and domestic violence—is shaped by subnational actors and eventually used in policy processes in Swiss cantons. International law is seen not only as a constraint but also as an opportunity and a resource that can be mobilized for different purposes by a variety of intermediaries, including local public officials, MPs, and members of civil society organizations, in collaboration and sometimes in competition with each other. Specifically, our findings indicate that international law intermediaries use these treaties through bottom-up dynamics of engagement according to their agenda and interests and their anticipation of what they can do in their cantonal context, resulting in iterative chains of intermediation. Furthermore, we observe a blurring of the boundaries between rule-makers, intermediaries, and targets, that is, the same actors may perform different roles in a given policy process.