Previous studies of environmental non-governmental organizations (ENGO) have primarily taken place within a nation-state perspective without considering multiple levels of politics and governance. Because environmental problems are usually cross-border phenomena, environmental movements must develop transnational features to play constructive roles in politics and governance. This study contributes to the theorizing and study of transnationalization of ENGOs by illuminating the different regional conditions for this process. The conditions for ENGOs to develop transnational collaboration are explored by comparing ENGOs from six countries in two macro-regions: Sweden, Germany, and Poland in the Baltic Sea region, and Italy, Slovenia, and Croatia in the Adriatic-Ionian Sea region. Grounded in the literatures on social movement theory and ENGO transnationalization, the study identifies how different national, macro-regional, and European institutional structures shape the conditions under which ENGOs develop cross-border collaborations, and demonstrate the importance of long-term and dynamic interplay between processes that occur at the domestic and transnational levels.