We develop an urban search-and-matching model. There is a central city, where all firms and jobs are located, and a continuum of peripheral cities. The population endogenously splits between migrants (who relocate from their hometown to the central city), commuters (who travel every day to work in the central city) and home stayers (who remain in their hometown). We prove that the market equilibrium is usually not optimal: a composition externality may generate under- or over-migration compared to the central planner’s solution, which results in under-investment in job vacancies and therefore production. We calibrate the model to the Greater Paris area and quantify this externality. Results suggest over-migration but policy interventions can help reducing inefficiencies.