A critical barrier to labor mobility arises from institutional constraints that im-pose discriminatory costs on migrants. Using China’s hukou system as a case study,we construct a novel, outcome-based measure of mobility frictions that infers thesediscriminatory costs. We document a systematic relationship between our frictionmeasure, migrants’ decisions to leave behind families (“partial migration”), remit-tances, and expenditure patterns. Our estimated spatial general equilibrium modelencompasses these features and examines how mobility frictions interact with tradeliberalization to shape migration, inequality, and welfare. Trade-exposed regionsbenefft from attracting migrants, while high-friction regions experience muted laborreallocation and smaller welfare gains.