Massive rural-urban migration and growing collective resistance are two profound transformations in China. This study examines how migration shapes collective resistance in sending areas. Drawing on several data sources, I find that migration acts as a vehicle of political diffusion and spurs collective resistance in rural China. But the role of migration differs by the form and scale of collective action and is conditioned by local social institutions. The role of migration can be understood in the context of distinct institutional arrangements in China, which were originally engineered to disenfranchise rural-origin people but which instead have inadvertently politicized migrants and peasants alike.