Children who are overweight or obese are at a higher risk of several diseases and are more likely to be overweight or obese in adulthood. Physical inactivity among children and adolescents is one of the major causes of the prevalence of overweight and obesity problems, and hence promoting physical activity, especially active school transport (AST), is a public health priority. Previous studies have provided evidence that urban greenery may promote physical activity and improve body weight outcomes among children. However, the relationships reported between these factors are inconsistent, partly due to the use of different measures of urban greenery. In the present study, we assessed different dimensions of urban greenery using three measures: number of parks; the Normalized Difference Vegetation Index (NDVI) based on satellite imagery; and street greenness based on Google Street View images. Multilevel regression analysis and structural equation modeling were conducted to investigate the complex relationships among urban greenery, AST, and body weight for 1,148 primary school students in Hong Kong. Overall, children attending schools with greener surrounding areas were more likely to engage in AST and had lower body mass index (BMI). In addition, AST partially mediated the impact of urban greenery on BMI. The findings demonstrate the importance of greenery around schools for children's body weight status. They also offer public health and planning guidance on how to address overweight and obesity problems among children.