Childhood obesity in the United States has disproportionately affected Latinx and Black children. The authors examine this issue by drawing upon implicit social cognition theory and social-psychological models of health and stress to propose and test a relation between negative implicit self-stereotyping and body mass index (BMI) and diet. Furthermore, it was predicted that self-esteem would buffer this relation because it is a psychological resource that functions to protect against stressors like the psychological experience of stigma. The authors recruited a community sample of 9-12-year-old Latinx and Black children and measured individual differences in implicit and explicit associations between the self and group stereotypes, self-esteem, objective BMI, and diet. Consistent with the main hypotheses, strong negative implicit (but not explicit) self-stereotyping was associated with higher levels of body mass indices in the obesity range and less healthy diet, but only among children with low self-esteem. Among children with high self-esteem, these relations were absent. These results held even after controlling for the contribution of parents' BMI, diet, education, and household income. These data are the first to theoretically and empirically link implicit self-stereotyping and self-esteem with physiological risk factors for chronic health conditions. Thus, this research contributes to understanding disparities among stigmatized ethnic-racial children in the United States and beyond.