Using peer volunteers as delivery agents may improve translation of evidence-based physical activity promotion programs for older adults. This study examined whether tailored support from older peer volunteers could improve initiation and long-term maintenance of physical activity behavior.;;Participants were randomized to 2 16-week, group-based programs: (1) peer-delivered, theory-based support for physical activity behavior change; or (2) an intervention typically available in community settings (basic education, gym membership, and pedometer for self-monitoring), attention-matched with health education. Moderate-to-vigorous physical activity (MVPA) was assessed via daily self-report logs at baseline, at the end of the intervention (16 weeks), and at follow-up (18 months), with... accelerometry validation (RT3) in a random subsample.;;Seven peer volunteers and 81 sedentary adults were recruited. Retention at the end of the trial was 85% and follow-up at 18 months was 61%. Using intent-to-treat analyses, at 16 weeks, both groups had similar significant improvements in MVPA. At 18 months, the group supplemented with peer support had significantly more MVPA.;;Trained peer volunteers may enhance long-term maintenance of physical activity gains from a community-based intervention. This approach has great potential to be adapted and delivered inexpensively in community settings.