This paper integrates literature on social movements and migration to examine how migration shapes both the cognitive and social foundations of collective action in origin communities.Using longitudinal data and in-depth interviews from rural China, we find that outward migration spurs collective resistance in origin communities and shapes the form and scale of collective action. Migration fosters non-institutionalized rather than institutionalized collective action, because it induces relational diffusion that empowers peasants to mobilize and employ more effective resistance strategies. This holds more for small-and medium-size collective action than for large-scale mobilizations, mainly because out-migration can also trigger community disintegration that inhibits larger-scale action. Furthermore, local social institutions condition the role of migration: migration has a stronger positive impact in close-knit villages embedded in strong lineage networks than in less cohesive villages. We contextualize the findings against the distinct institutional arrangements in China, which were originally engineered to disenfranchise rural-origin people but which instead have inadvertently politicized migrants and peasants alike.