The relationship between nationalism and public policy remains largely unexplored. Focusing on the link between sub-state nationalism and social policy, this article formulates three main arguments. The first is that social policy is likely to factor into processes of identity- and nation-building spearheaded by sub-state nationalism, and that nationalist movements typically trigger pressures for the decentralization of social policy. The second is that recognizing the importance of this connection should not lead to hasty conclusions about the impact of societal and institutional pressures on public policy. Nationalism is not only a societal force; it is also a political construction that reflects broad institutional legacies. The third is that the historical sequence of nationalism and policy development may create institutional forces preventing the congruence between policies and ethno-linguistic boundaries, in spite of pressures emanating from nationalist movements. The analysis of Social Security politics in contemporary Belgium provides ground to these claims.