How does political imprisonment influence anti-regime protest? We argue that political imprisonment facilitates rather than stifles protest. Political imprisonment is a salient indicator of arbitrary rule creating ‘embodied grievances’. It enables the formation of dissident networks within prisons, and serves as a legitimating credential for former inmates to lead resistance. These mechanisms imply that political imprisonment is a self-defeating strategy, making it easier for the opposition to overcome their collective action problem. We test our argument with subnational data from the German Democratic Republic between 1984 and 1989. To account for endogenous latent dissent, we use originally collected archival data on local surveillance operations. Exploiting daily variation in the timing of protests in 1989, we analyze the long-term impact of political imprisonment on mobilization. Results from survival analyses lend support to our hypothesized relationship. Combined with semi-structured interviews to probe our mechanisms, our findings suggest that political imprisonment increases the likelihood of protest mobilization.