--- - |2 Abstract When religious differences are present within an ethnic group, how do they affect the scope of its nationalist mobilization? The Kurds of Iran presents an ideal case to address this question given their religious diversity and varying levels of involvement in Kurdish nationalist movements. Building on an institutional approach to ethnic identity, this article argues that the dynamics of Kurdish ethnic mobilization in Iran reflect the nature of political exclusion in the Islamic Republic that is primarily based on sectarian affiliation. The article, based on original datasets compiled using several languages, including Persian and Kurdish, shows that recruitment into the Kurdish insurgency in Iran is significantly stronger in the Sunni Kurdish areas than the Shiite ones. While religious identity limits the appeal of ethno‐nationalism among the Shiite Kurds, it doubles the sense of marginalization among the Sunni Kurds and makes them more receptive to violent insurgent mobilization. - 'Nations and Nationalism, EarlyView. '