The process of modern social development is determined by two dialectically opposite tendencies: centripetal, expressed in worldwide integration and globalization, and centrifugal, manifested in regional localization, one of the forms of which is the phenomenon of separatism – the desire of regions within states to separate, to acquire greater political independence, sovereignty up to territorial separation or joining another state. The purpose of this paper was to analyse the theoretical issues of national self-determination and the principle of equality of peoples in its relationship with separatism. The study used a set of methods and techniques of scientific cognition. The principal ones included the dialectical method, comparative legal method, formal legal method, and method of logical analysis. The paper analyses the possibility of reference to the principle of self-determination of peoples by separatist entities in Ukraine as a justification for their activities. The minimum necessary criteria of legal grounds for self-determination are considered, including the presence of an effective connection of the subject of self-determination with a certain territory; the presence of the subject itself, i.e., the people (ethnos), which claims self-determination; and recognition by the international community of such a potential subject as the bearer of the right to self-determination. As for the connection with the territory, the doctrine of international law and practice recognizes the right to cultural and national self-determination in a certain territory for any ethnic group. This right is limited to the common interests of the entire people of the state, which lies in the unconditional preservation of the inviolability and integrity of its territory. As for the presence of the subject of self-determination itself, the people recognize the historical community of people that has developed in a certain territory and has stable features of language, culture, and mentality, as well as the awareness of their unity and difference from others through conscious self-name. At the same time, diasporas and migrants must obey the laws of the country of residence on the right of citizenship or the right of permanent residence. As for the recognition of the subject of self-determination by the international community, they recognize those who are under colonial rule, occupation by a foreign state, or who are subject to a policy of racism. Consequently, international law does not protect separatist movements whose goal is secession if they do not meet these criteria. This means that separatist entities in Ukraine are not entitled to secession, and by their methods of activity they are terrorist organizations. The practical value of the materials and conclusions presented in this paper lies in their further use in further developments aimed at the analysis and study of the self-determination of peoples through separatism, in the law-making sphere, and the sphere of law enforcement related to ensuring the rights and freedoms of citizens.