This article examines the transnational solidarity campaign for Francisco Ferrer, the Catalan anarchist and educator who was sentenced to death for his alleged involvement in Barcelona’s 'Tragic Week' of 1909. The international scale of the protests against Ferrer's execution was much remarked upon by his contemporaries. While historians have examined both the nature of demonstrations in support of Ferrer and the way in which he was commemorated, they have mostly focused on specific national contexts. This article takes a different approach: it investigates the transnational dimensions of the campaign. It places the protests within the framework of the 'culture wars' surrounding church–state relations. These cleavages were inherently transnational, and the structures developed by the international freethought movement, for example, played a significant role in sustaining the Ferrer campaign. The article also draws attention to other factors that shaped the protests and transcended national categories: from widespread images of Spanish 'despotism' to the way in which a foreign case could be adopted for domestic political mobilisation.