Background: Self-harm is a significant public health concern during adolescence. Most cases of self-harm do not come to the attention of professional services, however, where adolescents do disclose self-harm, the majority confide in their friends. Objective: Therefore, more needs to be understood about the protective function and risks friends may pose in the context of self-harm and personal implications of this behaviour. This review synthesises current research in this area. Methods: A systematic search of five healthcare databases was conducted, yielding nine papers, comprising of ten studies. Most studies recruited participants from schools. Papers were appraised using three quality appraisal tools relevant to study designs. Results: The synthesis found that friends act as first-line support for adolescents who self-harm, demonstrating emotional and physical availability and acting as a vehicle for terminating this behaviour and disclosure to adults. However, supporting friends who self-harm can be all-consuming, negative peer dynamics may perpetuate self-harm and group and personal vulnerability factors may increase risk of friends engaging in self-harm themselves. Conclusions: There are some indications that adolescents who self-harm may self-select friendship groups in which this behaviour is known to occur. The synthesis suggests adolescents need to be empowered to support peers who self-harm through mental health promotion and awareness-raising training at universal levels (e.g., whole school approaches). Leads for mental health in schools may also be important contact points for adolescents to disclose concerns to and seek personal support. Further practice and research implications are considered.