Background: Physical activity has been inconsistently associated with risk of preterm birth, and the strength of the association and the shape of the dose-response relationship needs clarification. Objectives: To conduct a systematic review and dose-response meta-analysis to clarify the association between physical activity and risk of preterm birth. Search strategy: PubMed, Embase and Ovid databases were searched for relevant studies up to 9 February 2017. Selection criteria: Studies with a prospective cohort, case-cohort, nested case-control or randomized study design were included. Data collection and analysis: Data were extracted by one reviewer and checked for accuracy by a second reviewer. Summary relative risks (RRs) were estimated using a random effects model. Main results: Forty-one studies (43 publications) including 20 randomized trials and 21 cohort studies were included. The summary RR for high versus low activity was 0.87 [95% confidence interval (CI): 0.70-1.06, I2 = 17%, n = 5] for physical activity before pregnancy, and it was 0.86 (95% CI: 0.78-0.95, I2 = 0%, n = 30) for early pregnancy physical activity. The summary RR for a 3 hours per week increment in leisure-time activity was 0.90 (95% CI: 0.85-0.95, I2 = 0%, n = 5). There was evidence of a nonlinear association between physical activity and preterm birth, Pnonlinearity < 0.0001, with the lowest risk observed at 2-4 hours per week of activity. Conclusion: This meta-analysis suggests that higher leisure-time activity is associated with reduced risk of preterm birth. Further randomized controlled trials with sufficient frequency and duration of activity to reduce the risk and with larger sample sizes are needed to conclusively demonstrate an association.