This article builds on previous research on Europeanization and political opportunity structures (POS) for mobilization, to explore the processes of transnational LGBT (Lesbian, Gay, Bisexual, and Transgender) mobilization in the European Union (EU). In the case of LGBT activism, European integration affects contentious politics by altering POS – both vertically and horizontally – for mobilization and changing the tactics of LGBT activism. Using the cases of Germany and Poland to trace cross-border connections between norm entrepreneur and target state, the findings suggest that LGBT activism relies on transnational resources – primarily, social spaces and organizational capacity – that are scarce in many member states but readily available in others. These horizontal opportunities among member states, alongside top-down vertical ones provided by Brussels, serve as mobilizing structures that bring together distinct groups of international actors. Europeanization also alters the tactics that transnational actors use when engaging with authorities in the target state. Employing socialization mechanisms that highlight appropriate behavior, actors tactically frame their demands in a European discourse by associating the issue of LGBT acceptance with democratic responsibilities as members of the EU community.