This paper aims to point to the transition from international law to transnational law that, on the one hand, is caused, and on the other, is strengthened by the growing role of cities in the fight against COVID-19. Various interactions between cities and other international actors give rise to new trends and challenges on the international plane. One of such terms, transnational law, refers to developments beyond the nation-state and includes “all law which regulates actions or events that transcend national frontiers”. It is characterized by a plurality of overlapping normative systems and a growing role of new actors in the international arena, which are cities. The authors give examples of cities bypassing or complementing states with special emphasis on European cities (Polish including) as well as of cities’ transnational cooperation to fight COVID-19 pandemic, filling the gaps in inter-governmental multilateral cooperation.