Background: Postpartum weight gain is a well-known challenge for many mothers, but associations with green space quantity and quality have not been investigated. Methods: This longitudinal study used data on 3843 mothers living in Australia tracked biennially for 15 years post partum from 2004 onwards. Multilevel growth curve models adjusted for confounding were used to examine the patterning of body mass index (BMI) in relation to green space quantity, measured by percentage land use, and green space quality, measured by self-report. Two-way interaction terms were fitted to investigate time-contingent associations between BMI and green space. Results: Compared with mothers in areas with ≤5% green space, adjusted BMI coefficients were -0.43 kg/m2 (SE 0.37), -0.69 kg/m2 (SE 0.32) -0.86 kg/m2 (SE 0.33) and -0.80 kg/m2 (SE 0.41) among mothers in areas with 6%-10%, 11%-20%, 21%-40% and ≥41% green space, respectively. There were no independent associations between BMI and green space quality. Evidence suggested mothers living in areas with 21%-40% green space had the lowest BMI, whether they agreed that local parks were good quality (-0.89 kg/m2 (SE 0.34)) or not (-0.93 kg/m2 (SE 0.35)). Mothers in the greenest areas only had statistically significantly lower BMI if they perceived local parks as high quality (-0.89 kg/m2 (SE 0.41)). There was limited evidence that these associations varied with respect to the number of years post partum. Conclusion: These findings may suggest that urban greening strategies to achieve a threshold of at least 21% or more green space in an area may help reduce, but not fully prevent postpartum weight gain. Potential mechanisms warrant investigation.