Background:Postpartum women are encouraged to participate in ≥ 150 min/week of moderate-intensity aerobic activity, but few women achieve this recommendation. This study sought to identify factors associated with participation in physical activity after pregnancy.Methods:We examined correlates of any self-reported moderate-to-vigorous physical activity (MVPA) (≥ 10 min/week across all modes) and any recreational MVPA (≥ 10 min/week) among women enrolled in the Pregnancy, Infection, and Nutrition Postpartum study at 3 months postpartum (n = 667) and at 12 months postpartum (n = 530). Potential correlates were identified according to the socioecological framework.Results:At 3 and 12 months postpartum, lower odds of participation in any MVPA were associated with lower education, breastfeeding, and minimal emotional support. Low exercise self-efficacy, receipt of advice about physical activity, and warmer seasons were associated with higher odds of any MVPA. For recreational MVPA, lower odds of participation were associated with unmarried status, lower education, employment, low income, preeclampsia, and minimal emotional support. Involvement in child/adult care activities, transportation MVPA, and warmer seasons were associated with higher odds of recreational MVPA.Conclusion:These findings suggest that several modifiable intrapersonal and interpersonal factors are associated with postpartum MVPA and should be considered when developing interventions to help women maintain or increase MVPA after pregnancy.