<p>What effect, if any, does information about other people’s attitudes (peer opinion) have on legitimacy beliefs toward international organizations (IOs)? While previous research has identified multiple sources of IO legitimacy beliefs, it has so far overlooked the potential role of peer opinion. This article provides the first systematic analysis of peer opinion effects on people’s legitimacy beliefs toward IOs. To this end, it develops theoretical expectations about the effects of peer opinion and the conditions under which these effects are likely to be particularly strong. It tests these hypotheses through three survey experiments administered to nationally representative samples in Brazil, Germany, and the United States, each experiment evaluating one potential source of peer opinion: other survey participants, national polls, and social media threads. The article finds that peer opinion shapes legitimacy beliefs toward IOs across experiments and that this effect tends to be particularly strong when peer opinion is negative and reflects a high level of consensus. We also find peer opinion effects to be surprisingly stable under conditions of higher political awareness and elite cues working in the opposite direction. These findings suggest that people form legitimacy beliefs toward IOs through a process that is more social and horizontal than previously understood.</p>