PURPOSE Peer support can promote diabetes control, yet research on feasible and effective peer support models is lacking. This randomized controlled trial tested a volunteer-based model of peer support for diabetes control. METHODS Thirty-four volunteer peer leaders were recruited and trained to provide support to 5 to 8 patients each through telephone contact, in-person, individual, and group support. Planned dose was 8 contacts, preferably in the first 6 months. Patients with uncontrolled diabetes were randomly sampled from the medical records of 3 community clinics. After a baseline interview and medical records review to obtain baseline values for the primary outcome, HbA1c, 336 patient participants were randomly assigned to a 12-month peer support intervention or usual care. The assessment protocol was repeated at 6 and 12 months after baseline. RESULTS Thirty peer leaders delivered an average of 4 contacts each per assigned participant (range 1-24). Despite the lack of intervention fidelity, the intervention was effective at reducing glycated hemoglobin (HbA1c) among intervention as compared with usual care participants (P=0.05). Similar trends were observed in frequency of meeting fruit and vegetable guidelines (P =0.09), a secondary outcome. Counterintuitively, usual care participants reported checking their feet more days out of 7 than intervention participants (P =0.03). CONCLUSIONS Given the modest changes we observed, combined with other evidence for peer support to promote diabetes control, additional research is needed on how to modify the system of care to increase the level of peer support delivered by volunteers.