City diplomacy has turned to new searches in the 21st century to get results especially in the fight against global climate change, while it had a lower profile application area in international relations in the 20th century. The research question of this article, in the Age of Transboundary Crises, is to examine why cities need to be involved in solving global problems and their capacity to contribute in the U20 case study. This study aims to contribute to the international relations city diplomacy literature by examining Urban 20 (U20) with the case study method. The biggest difference of the 21st century from previous centuries is that the problems are transboundary and global, as in the examples of global warming and the Corona pandemic, and the motivation to act jointly takes priority because no international organization, state or other actors can solve these problems alone. In this article, brief information about city diplomacy and international local diplomacy organizations is given at first, and then, it is concluded that cooperation between cities can be beneficial in combating global problems with multi-layered stag hunt theory. It is also concluded that city diplomacy has a limited effect on international relations and does not have a legal sanction power.