Background Volunteers are increasingly promoted to improve health-related outcomes for community-dwelling elderly without synthesized evidence for effectiveness. This systematic review and meta-analysis evaluates the effects of unpaid volunteer interventions on health-related outcomes for such seniors. Methods MEDLINE, EMBASE and Cochrane (CENTRAL) were searched up to November 2018. We included English language, randomized trials. Two reviewers independently identified studies, extracted data, and assessed evidence certainty (using GRADE). Meta-analysis used random-effects models. Univariate meta-regressions investigated the relationship between volunteer intervention effects and trial participant age, percentage females, and risk of bias. Results 28 included studies focussed on seniors with a variety of chronic conditions (e.g., dementia, diabetes) and health states (e.g., frail, palliative). Volunteers provided a range of roles (e.g., counsellors, educators and coaches). Low certainty evidence found that volunteers may improve both physical function (MD = 3.2 points on the 100-point SF-36 physical component score [PCS]; 95% CI: 1.09, 5.27) and physical activity levels (SMD = 0.5, 95% CI: 0.14 to 0.83). Adverse events were not increased. Conclusion Volunteers may increase physical activity levels and subjective ratings of physical function for seniors without apparent harm. These findings support the WHO call to action on evidence-based policies to align health systems in support of older adults.