How do representative institutions diffuse from one polity to another? I investigate the effect of the Dominican order on the transition from autocratic to representative city government in medieval Europe. I argue that the order's practices of representation diffused to local lay politics because of the persistent interaction between Dominican monks and urban elites. Using a difference-in-difference design, I offer evidence that the presence of Dominican houses fostered the development of representative city government. My findings highlight the important role that religious institutions can play in the diffusion of political institutions and principles.