This study examines the effect of voters’ ideological extremism on turnout in European national and European Parliament elections. Using data from recent European Election Studies, the Comparative Study of Electoral Systems, and other national election studies, we find that, relative to centrists, ideological extremists (measured by self-placement on the left–right scales) are more likely to vote in European Parliament elections (2014 and 2019) but not national elections. We argue that these differences stem from the fact that European Parliament elections are second-order races. The results help to explain why the European Parliament has become more polarized, even in the absence of significant changes in overall attitudes among the European public, and why extreme parties have been more successful in recent European Parliament than national elections.