This article examines the role of regional norms and international customary law in Eastern Europe in assessing the legitimacy of the Russian Federation’s territorial claims over Ukraine. The central question is whether the recurrent patterns of conduct by states and regional organizations have been capable of creating a customary rule that could provide a legal basis for altering borders. The theoretical framework of the study relies on the two fundamental elements of customary international law—state practice and opinio juris—which are analyzed in connection with both regional and global responses. The research method is descriptive–analytical, based on documentary analysis of International Court of Justice judgments, United Nations resolutions, official state declarations, and relevant scholarly works. The findings indicate that, although certain recurring patterns of conduct can be identified at the regional level, the absence of effective consensus and the widespread opposition of the international community have prevented these practices from crystallizing into a binding customary rule. The article concludes by recommending the development of stronger legal and institutional mechanisms at both regional and global levels to reinforce non-recognition policies and to counter unlawful annexations.