Populism is usually treated as an exclusively top-down affair where political party leaders mobilize diverse constituencies to reap electoral benefits. This perspective discounts a rich universe of bottom-up populist mobilization that remains exogenous to strict electoral contestation, thus unreasonably constraining the empirical study of the phenomenon. This chapter draws from social movement studies and social psychology to examine populist social movements under a comprehensive theoretical framework, aiming to bring together theorists of populism with scholars of social mobilization and encourage their mutually beneficial interaction. It argues that populism—as a compelling political dialect—has traditionally informed and continues to inform significant waves of grassroots contention around the world, triggering seemingly extraordinary developments at the party system level while also potentially determining processes of democratization. The chapter concludes by predicting an increasing relevance for grassroots populism, urging scholars to widen their scope of study by embracing it alongside its top-down variant.