Purpose. This paper aims to show how the views of Kant persist in the modern debate on social justice and to outline the practical and political potential contained in his understanding of a just state system and international justice. To that end, I will present what Kant meant by a just state system and just relationships between states. Then, I will reference his understanding of social justice against three fundamental models of social justice thus far established in the philosophical tradition: (...) the legal, distributive, and contractual justice. Finally, I will explain how the Kantian understanding of social justice is reflected in select modern interpretations of a just state system and justice in international relationships, and how we can grasp the current practice and the expected development of sociopolitical life in the framework of social justice as understood by Kant. Originality. The article presents the theoretical-conceptual and practical-political relevance of the concept of social justice, developed by I. Kant. It was shown that Kant considered justice to be the basis of all correct social relations, both at the level of individual states and in the sphere of international relations. According to Kant, the only just state system is a republic. In his opinion, justice in the field of international relations requires that they be based on the principles of federal unification of individual states. The concept of social justice developed by Kant can be identified as a form of classically understood contractual justice. In addition, Kan’s notion of justice was correlated with the works of modern authors: John Rawls, Robert Nozick and Otfried Höffe, showing identical and different elements in them. Conclusions. The reflections above suffice to assert that the Kantian understanding of social justice primarily involves the following observations: 1. under a relevant contract, interested parties institute a public authority, i.e. a republican state or a federal institution endowed with judicial or executive powers ; 2. the institution of public authority remains fair in dealings with the governed; 3. the public authority has the right to intervene only in the circumstances of threat to the external freedom, equality and independence of the governed individuals; 4. the public authority is responsible only for guaranteeing just relations between individuals and the enforcement of their contracts. Furthermore, Kantian understanding of justice falls within the scope of the classical notion of contractual justice and remains present in the modern debates on justice, as exemplified by the thought of Rawls, Nozick and Höffe. Finally, the Kantian understanding of social justice remains topical and worthy of consideration during the design and the development of current and future solutions for a just public order, both at the national and international level. (shrink)