Purpose The purpose of this study is to report a systematic review of reviews of evidence and gaps focused on in-person and technology-mediated diabetes peer support and its impact on clinical, behavioral, and psychosocial outcomes. Methods We conducted a systematic review of reviews in accordance with the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) guidelines. Reviews published in English from December 1978 through December 2018 examining clinical, behavioral, and psychosocial outcomes were included. The search yielded 167 reviews that were examined for inclusion/exclusion criteria. Results Nine systematic reviews and meta-analyses meeting criteria were included. Findings suggest peer support interventions can have a positive impact on clinical (A1C, blood pressure, cholesterol, weight), behavioral (diabetes knowledge, being active, healthy eating, medication management, self-management, self-efficacy, empowerment), and psychosocial (social support, health and diabetes distress, depression, quality of life) outcomes. Research gaps exist related to understanding the effects of emerging technology-mediated peer support modalities and the effects of peer support on gestational diabetes. Conclusion Many clinical, behavioral, and psychosocial benefits related to in-person and technology-mediated peer support exist. Diabetes care and education specialists should incorporate and recommend peer support resources for people with diabetes.