A growing number of postsecondary institutions offer peer support interventions to promote mental health among their students. However, little is known about the key factors that influence the implementation of this type of intervention. This study explored barriers and facilitators to implementing an online peer support program based on Acceptance and Commitment Therapy aimed at reducing stress and anxiety among students of three universities in Montreal, Canada. Peer supporters were asked to complete a survey containing open-ended questions at the end of each semester. Interviews were also conducted with the program coordinator, three psychologists who acted as supervisors, and 14 peer supporters. The data were analysed with an inductive and a deductive approach using the Consolidated Framework for Implementation Research (CFIR). Perceived barriers to the implementation of the intervention included technical problems with the videoconferencing platform, psychosocial and comorbidity issues among peers, recruitment difficulties, and communication concerns between stakeholders. Key perceived facilitators were the theoretical and empirical foundations of the intervention, the support offered by directors, and the process used to recruit, select, and supervise peer supporters. Based on the results of this study, practical recommendations are provided for researchers and practitioners interested in implementing online peer support initiatives in postsecondary institutions.