While the site occupation at Wyhl in 1975 is usually considered the symbolic birthplace of the West German anti-nuclear movement, it may also serve as the starting point for a transnational history of anti-nuclear protest. Local cross-border cooperation among protesters at Wyhl deeply impressed those anti-nuclear activists in the mid-1970s who considered nuclear power a global problem and encouraged them to take their protest to the international level. The central argument of this article is that protest directed against international organizations (IOs) – notably the European Communities (EC) and the International Atomic Energy Agency (IAEA) provided a crucial catalyst for transnational cooperation among anti-nuclear activists. Targeting IOs as the key promoters of nuclear power on a global scale, anti-nuclear activists cooperated across borders organizing protest events. Their goal was to challenge the IOs and win back the public on the issue across borders. Based on multi-archival research in Western Europe, this article analyzes five transnational protest events between 1975 and 1978 in Europe. Findings suggest that continued cooperation led to the emergence of a transnational anti-nuclear network and facilitated transnational transfers of scientific expertise and protest practices.