Abstract Background Experiential peers are increasingly involved in the development and delivery of interventions for individuals who are engaged in delinquent behavior. Experiential peer support, which is the provision of support to an individual engaged in delinquent behavior by someone who has previously also practiced such behavior, is one such application. Little is known, however, about its effects on desistance, or the mechanisms that explain these effects. On the basis of theoretical papers, program descriptions, and interviews with experts, we developed an initial program theory. We propose seven mechanisms that might play a role in the (potential) effects of support by experiential peers: (1) empathy and acceptance, (2) social learning, (3) social bonding, (4) social control, (5) narrative and identity formation, (6) hope and perspective, and (7) translation and connection. In addition, in this protocol paper, we describe the methods of a systematic realist literature review that will be conducted in order to investigate the evidence base for this program theory. Methods The study described in this protocol paper is a realist review, which is a suitable approach to study complex interventions and fits the explanatory purpose of the study. We outline the steps to be taken for the systematic realist review, including the selection and assessment of studies and the methods for synthesizing the findings. Discussion Investigating the effects and the underlying mechanisms of support by experiential peers for individuals with delinquent behavior is relevant because the forensic setting has some unique features, and the involvement of service users might create even more tension than in other settings due to stigma and perceived risks. The findings that will be reported in the realist review will contribute to the knowledge of the effects of support by experiential peers and will provide insight into which aspects remain to be studied. It might also provide formal care institutions with guidance on whether to involve experiential peers in the delivery of their services and the conditions under which these interventions are likely to lead to positive results.