<p>How do voters’ and political parties’ characteristics on populism affect voters’ estimations of political parties’ left/right ideologies? Following the theories of cognitive projection in social psychology, we argue that the similarity or disparity in populism between voters and political parties affects voters’ estimations of political parties’ left/right ideologies. Using survey and party position data in 15 Western European countries, we find that people locate a political party’s left/right ideological position closer to their own stance than it is in reality when their populism position is similar with the political party’s populist ideology though the substantive size of the projection bias effect is not very large. We also find that this populism effect is larger among people with a low education level, people without political interest, and political parties with an ambiguous left/right ideological position. The results imply cognitive connections between populism and left/right ideology and help us to understand political behaviors that crosscut the traditional left/right ideological dimension.</p>