Legitimacy is central to the functioning of global governance institutions (GGIs) such as the European Union (EU) and the United Nations. There is a vibrant debate about legitimacy in International Relations, and a burgeoning literature in comparative politics on public attitudes towards the EU. Yet, these literatures rarely speak to each other, which has resulted in missed opportunities for theoretical advancements on the sources and consequences of citizens’ legitimacy beliefs vis-à-vis GGIs. To assist researchers in advancing on this state of the art, this research note develops a conceptualization of popular legitimacy as a multidimensional belief system including both moral convictions and self-interest. A statistical analysis of public attitudes towards the EU from 1973 to 2012 suggests that commonly used survey measures capture self-interest rather than moral beliefs. This note concludes by suggesting a research agenda intended to push theory and survey research on legitimacy beliefs towards GGIs forward.