Key PointsQuestionAre body mass index (BMI)zscores and waist circumference associated with brain integrity and connectivity among preadolescent children?FindingsIn this cross-sectional study of 4576 children, higher BMI and waist circumference were associated with reduced white matter (WM) integrity and neuronal density, lower cortical thickness, and decreased functional connectivity of cognitive control– and reward-related networks. Over a 2-year follow-up period, higher baseline BMIzscores were associated with lower interval development of several commissural, projection, and association WM tracts as well as the prefrontal cortex.MeaningIn this study, higher childhood BMI and waist circumference were associated with poorer imaging metrics of brain integrity and hindered interval evolution of WM microstructure, WM cytostructure, and cortex morphology.