Despite recent calls for more peer support initiatives aimed at promoting mental health in postsecondary institutions, those initiatives remain scarce. In this study, a multisite randomized controlled trial was designed to assess the effect of an online peer support intervention based on acceptance and commitment therapy using mental health and school indicators. Undergraduate students were recruited in three Canadian universities and randomly assigned to an intervention (n = 54) or a wait-list control group (n = 53). Compared to control participants, those who took part in the program self-reported reduced psychological inflexibility, stress, anxiety and depression, and increased psychological flexibility and well-being. The intervention had no effect on academic satisfaction and engagement. These results were found both in completer and intent-to-treat samples. The findings provide evidence that peer support may be a beneficial adjunct to mental health interventions offered to college and university students.