Aim. To identify positive trends and problematic issues in the protracted process of harmonizing the social rights of citizens of Belarus and Russia in the context of citizenship of the Union State. Methodology. The study was carried out using traditional methodological tools (a systematic approach, comparative legal analysis, statistical, formal legal and other methods). Results. The article presents the author’s judgments about the criteria of social statehood, the nature of citizenship of interstate entities, the Union State of Belarus and Russia; the conclusion is substantiated about the mainly international legal (but not constitutional-legal) basis of integration processes in relations between the Russian Federation and the Republic of Belarus, including in the sphere of ensuring equal rights of citizens. Research implications. The presented material supplements the theory of constitutional law, in particular, in such areas of scientific knowledge as the theory of the status of the individual, social rights and social statehood, the internationalization of constitutional law and the constitutionalization of interstate relations.