Objectives The purpose of this study was to estimate a height‐BMI association in child and female adolescent populations in developing countries, and to evaluate the potential role of relative economic status in this association. Methods Flexible structured additive regression models were used to estimate associations between height, BMI, and relative wealth. Linear regression models were used to evaluate height‐wealth interactions on BMI outcomes. The models were applied to pooled samples of 971,180 children under 5 years of age and 225,718 adolescent females between 15 and 19 years. Samples were taken from Demographic and Health Surveys from 64 developing countries. Results Children exhibited a strong inverse association between height and BMI over the entire distribution of height. Female adolescents showed a weak inverse association. Relative economic status was associated with a stronger height‐BMI relationship in early‐life (<3 years) and a weaker relationship as children age into their fifth year. Conclusions Relative economic status may protect against consequences of a negative height‐BMI association in young children in developing countries, first by promoting higher body weight among shorter children during the risk period for early‐life mortality, and afterwards by limiting body weight in shorter children when overweight and obesity become longer‐term health issues.