The 2019 European Parliament (EP) elections produced remarkable gains for Green parties throughout the European Union (EU). We analyse in how far different national electoral contexts can explain the electoral success of Green parties in EU member states. On the theoretical side, we argue that an increased salience of EU environmental politics has led to 'green issue voting'. However, the relevance of environmental issue preferences depends on the electoral context in which Green parties compete for votes. Specifically, we expect 'green issue voting' to be most relevant for the smaller Green parties who own the environmental issue. On the empirical side, we test our argument by combining survey data collected by the European Election Study with information on party positions provided by the Comparative Manifestos Project. Our findings strongly suggest that the relevance of 'green issue voting' in the 2019 EP elections was conditionally dependent on the national electoral context.