Why do sub- and non-state actors participate in transnational climate governance? Existing explanations tend to focus on international processes of diffusion and the micro-incentives facing individual actors. These approaches frequently assume, often implicitly, that the domestic contexts in which cities, NGOs, firms, and other actors are embedded do not shape their engagement in transnational governance. Often, this is because scholars have restricted their analyses to wealthier, liberal democratic countries. Here, we argue that national context plays a pivotal role in actors’ decisions to participate in transnational governance schemes. The paper uses a new dataset of cross-national participation by sub- and non-state actors in transnational climate governance to evaluate this argument. We find substantial cross-national variation in actor participation in transnational climate initiatives, and robust evidence that both domestic institutions and policies systematically shape patterns of such engagement. The findings suggest that domestic politics “matter” for transnational governance, and indicate that participation in TCG will be greatest in countries with strong civil liberties, decentralized government, and pro-climate policies.