The recent rise of populist politicians in Western democracies is often associated with their allegedly successful use of digital media. However, for a long time, there has been little research specifically on populist online communication. To address this substantial research gap, the thesis pursues two major research aims: First, it investigates drivers of populist communication in politicians’ online self-presentation and online news media representation. Second, the thesis examines the effects of populist online communication on citizens’ behavior in the form of user reactions to politicians’ social media posts and reader comments on online news articles. Based on five internationally comparative studies and the overarching synopsis, the cumulative thesis demonstrates that populist online communication is driven by the reciprocal interactions among politicians, journalists, and citizens and is influenced by various factors on the macro, meso, and micro level. Furthermore, it shows that populist online communication resonates with citizens and is multiplied by them – specifically by citizens with prior strong populist attitudes. By analyzing the interactions of three key actor groups – politicians, journalists, and citizens – and by following a multimethod approach the dissertation connects research on both the supply and demand side of populism.