AbstractLow socioeconomic status (SES) in childhood is associated with deficits in executive function and changes in cortical morphology. Furthermore, rates of childhood obesity are greater among low SES children and childhood obesity is also associated with cortical alterations and impaired neurocognition, specifically in the domain of executive function. To investigate the influence of BMI on the relationships between SES and both neurocognition and brain morphology, we used data from the Adolescent Brain Cognitive Development (ABCD) study to construct multiple linear regression models and conduct mediation analyses. Overall, SES as measured by household income, highest level of parental education, and area deprivation, was associated with lower BMI, greater total and prefrontal cortical volume, and better performance on assessments of executive function. Mediation analysis indicated that BMI had a significant indirect effect on associations between area deprivation and both total and prefrontal cortical volumes. BMI also played a mediating role in the associations between area deprivation and composite neurocognitive scores, which were driven by performance on tasks of working memory and cognitive flexibility, but not cognitive control. These findings suggest that BMI should be considered in future studies investigating the relationship between low SES and poor neurodevelopmental outcomes.