In recent years, both diasporas and cities have become important non-state actors in international relations. This development has spurred new subcategories of diplomacy, i.e. “diaspora diplomacy” and “city diplomacy.” However, there is a lack of research and publications linking the diplomatic activity of these two actors. The article presents a case study of international cooperation developed between various sub-state actors from Poland and Italy. This collaboration was initiated and jointly implemented by a Polish diaspora organization, with the support of the Polish dip-lomatic corps. The study aims to show the dynamic interactions and benefits arising from collaboration between different actors operating at the intersection of public, cultural, and economic diplomacy. The article consists of three main sections. In sec-tion one, we define the concepts of “diaspora diplomacy” and “city diplomacy.” The second section presents a description of activities undertaken as part of the analyzed project. In the third section, we present the role and importance of the key stakehold-ers involved in this initiative, highlighting the outcomes and benefits of cross-sector collaboration. The project is co-financed by funds from the state budget, granted by the Minister of Education and Science of Poland under the “Excellent Science II” Program.