Background Outdoor air pollution is increasingly recognised as a key threat to population health globally, with particularly high risks for urban residents. In this study, we assessed the association between residential nitrogen dioxide (NO₂) exposure and children’s cognitive and behavioural development using data from São Paulo Brazil, one of the largest urban agglomerations in the world. Methods We used data from the São Paulo Western Region Birth Cohort, a longitudinal cohort study aiming to examine determinants as well as long-term implications of early childhood development. Cross-sectional data from the 72-month follow-up was analysed. Data on NO₂ concentration in the study area was collected at 80 locations in 2019, and land use regression modelling was used to estimate annual NO₂ concentration at children’s homes. Associations between predicted NO₂ exposure and children’s cognitive development as well as children’s behavioural problems were estimated using linear regression models adjusted for an extensive set of confounders. All results were expressed per 10 μg/m³ increase in NO2. Results 1143 children were included in the analysis. We found no association between NO₂ and children’s cognitive development (beta -0.05, 95% CI [-0.20; 0.10]) or behavioural problems (beta 0.02, 95% CI [-0.80; 0.12]). Conclusion No association between child cognition or child behaviour and NO₂ was found in this cross-sectional analysis. Further research will be necessary to understand the extent to which these null results reflect a true absence of association or other statistical, biological or adaptive factors not addressed in this paper.