The full-scale Russian aggression against Ukraine has increased the importance of legal argumentation in international political communication and transformed public diplomacy into an important instrument of international legitimisation and strategic mobilisation. Under these conditions, the legal public diplomacy of the President of Ukraine acquired particular significance in strengthening international democratic solidarity and support for the rules-based international order, including within Ukrainian-American relations. The aim of the article was to determine the principal instruments, functions and political significance of the legal public diplomacy of the President of Ukraine during the full-scale Russian aggression and to clarify its role in strengthening international democratic and legal solidarity, with particular emphasis on the Ukrainian-American dimension. The methodological framework combined qualitative content analysis, discourse analysis, framing analysis and the comparative method. It has been established that legal discourse became one of the central components of the wartime public diplomacy of the President of Ukraine. References to international law, sovereignty, territorial integrity, war crimes and international accountability were systematically employed to legitimise Ukraine’s position and delegitimise Russian aggression. The study demonstrated that legal public diplomacy contributed to strengthening democratic solidarity between Ukraine and the United States by framing the war as a global challenge to international legality and collective security. It has also been determined that legal public diplomacy functioned as a distinct form of strategic communication combining legal legitimisation, political mobilisation and strategic narrative construction. The practical significance of the study consisted in the possibility of applying its findings within international relations, strategic communication, public diplomacy and international law for analysing legal narratives and mechanisms of international legitimisation during contemporary armed conflicts