Lower neighbourhood-level socioeconomic status (SES) has been repeatedly associated with an increased risk of adverse birth outcomes, even after controlling for individual-level SES. Few studies have empirically assessed potential mechanisms underlying the associations. The objectives of this study were to (1) examine relations between neighbourhood SES and birth outcomes, and (2) explore if maternal weight variables mediated these relations. Data came from a provincial prospective pregnancy cohort study in Canada. Census data was used to create a continuous measure of neighbourhood SES. Using information from maternal questionnaires and medical records, two mediators (pre-pregnancy body mass index (BMI), and gestational weight gain (GWG)) and five birth outcomes (preterm birth, low birth weight, macrosomia, small for gestational age (SGA), large for gestational age (LGA)) were examined. After adjusting for individual-level covariates, mediation analyses supported significant associations between lower neighbourhood SES and increased risk of macrosomia (b = 0.1183, 95% BCa CI: 0.0607-0.1896) and LGA (b = 0.0565, 95% BCa CI: 0.0040-0.1186) through higher pre-pregnancy BMI. Significant associations were also observed between neighbourhood SES and macrosomia, LGA, and preterm birth (b = 0.0105, 95% BCa CI: 0.0014-0.0246) through pre-pregnancy BMI and GWG in tandem; pairwise comparisons suggested that associations with macrosomia and LGA through pre-pregnancy BMI alone were significant over associations through pre-pregnancy BMI and GWG together. These findings add to a growing body of literature assessing potential mechanisms underlying relations between neighbourhood SES and adverse birth outcomes, and suggest that neighbourhood-level SES may influence birth outcomes through maternal weight.