Urban environments are characterized by multiple exposures that may influence body mass index (BMI) growth in early life. Previous studies are few, with inconsistent results and no evaluation of simultaneous exposures. Thus, this study aimed to assess the associations between exposure to air pollution, green spaces and built environment characteristics, and BMI growth trajectories from 0 to 5 years. This longitudinal study used data from an electronic primary care health record database in Catalonia (Spain), including 79,992 children born between January 01, 2011 and December 31, 2012 in urban areas and followed until 5 years of age. Height and weight were measured frequently during childhood and BMI (kg/m2) was calculated. Urban exposures were estimated at census tract level and included: air pollution (nitrogen dioxide (NO2), particulate matter <10 μm (PM10) and <2.5 μm (PM2.5)), green spaces (Normalized Difference Vegetation Index (NDVI) and % green space) and built environment (population density, street connectivity, land use mix, walkability index). Individual BMI trajectories were estimated using linear spline multilevel models with several knot points. In single exposure models, NO2, PM10, PM2.5, and population density were associated with small increases in BMI growth (e.g. β per IQR PM10 increase = 0.023 kg/m2, 95%CI: 0.013, 0.033), and NDVI, % of green spaces and land use mix with small reductions in BMI growth (e.g. β per IQR % green spaces increase = −0.015 kg/m2, 95%CI: −0.026, −0.005). These associations were strongest during the first two months of life. In multiple exposure models, most associations were attenuated, with only those for PM10 and land use mix remaining statistically significant. This large longitudinal study suggests that early life exposure to air pollution, green space and built environment characteristics may be associated with small changes in BMI growth trajectories during the first years of life, and that it is important to account for multiple exposures in urban settings. Display omitted •Large longitudinal study evaluating multiple urban exposures.•Air pollution, green spaces and built environment were associated with small changes in early life BMI trajectories.•Associations were strongest during the first two month of life.•Important to take into account multiple exposures in urban settings.