The article analyses the work of the Fédération internationale de la Libre Pensée, an organization that was founded in 1880 as a transnational actor in Europe's 'culture wars'. With a secretariat based in Brussels, the federation organized twenty-five international congresses in different cities and countries between 1880 and 1938, bringing together representatives of secularist groups as well as political activists and scientists through its events. The federation hence played a role in creating transnational meeting spaces — yet it could also serve as a campaigning tool, reflected by its role in the international solidarity campaign for the Catalan educator and anarchist Francisco Ferrer. Significantly, the federation provided a forum for both socialists and liberals, who were unified in identifying the role of organized religion (in particular the Catholic church) as a matter of transnational concern. By tracing the main features of this 'Freethinkers' International' and its constituency, the article questions the boundaries between different forms of internationalism. At the same time, the relation between 'freethought' and 'socialism' remained a contentious issue for the federation both before and after World War I, demonstrating the obstacles to further-reaching transnational cooperation.