Background: There is currently no specific guidance on the role of exercise in managing postpartum depression in the UK and US, and international guidance is inconsistent. Aim: To assess the effectiveness of aerobic exercise on postpartum depressive symptoms. Design and setting: Systematic review and meta-analysis. There was no restriction to study site or setting. Method: The databases MEDLINE, EMBASE, Cochrane Library, PsycINFO, SportDiscus, ClinicalTrials.gov, and the World Health Organization International Clinical Trials Registry Platform were searched. Titles and abstracts, then full-text articles, were screened against inclusion criteria: RCTs measuring depressive symptoms in mothers ≤1 year postpartum; and interventions designed to increase aerobic exercise compared with usual care or other comparators. Included studies were assessed using the Cochrane Collaboration's risk of bias tool. Meta-analysis was conducted. Pre-planned subgroup analyses explored heterogeneity. Results: Thirteen RCTs were included, with 1734 eligible participants. Exercise significantly reduced depressive symptoms when all trials were combined (standardised mean difference -0.44; 95% confidence interval = -0.75 to -0.12). Exploration of heterogeneity did not find significant differences in effect size between women with possible depression and in general postpartum populations; exercise only and exercise with co-interventions; and group exercise and exercise counselling. Conclusion: This systematic review provides support for the effectiveness of exercise in reducing postpartum depressive symptoms. Group exercise, participant-chosen exercise, and exercise with co-interventions all may be effective interventions. These results should be interpreted with caution because of substantial heterogeneity and risk of bias. Keywords: depression; depression, postpartum; exercise; postnatal; review, systematic.