["Sociological Forum, EarlyView. ", "\nABSTRACT\nSolidarity is a core concept in sociology. The foundational work on solidarity was fundamentally concerned with questions of historical transformation. By contrast, postwar sociological theorizing has tended to take national solidarity as the concept's final and definitive form. This methodological nationalism is poorly suited to explaining and interpreting solidarity in a period marked by world risk. Given contemporary global social problems, it is necessary to re‐historicize and re‐theorize solidarity. This article develops a novel historical‐theoretical account that traces the epochal shifts in the relationship between solidarity and ontological uncertainty. This account concludes with the observation of a new “post‐national solidarity” characterized by a sense of pervasive uncertainty and a declining faith in national‐level institutions. At the same time, this shift has provoked a wave of re‐nationalization, as political actors seek to reclaim solidarity within bounded national frameworks, often in reaction against the cosmopolitan aspirations of post‐national solidarities.\n"]