Research has shown that parental separation is associated with worse physical health and unhealthy weight gains during childhood. However, limited empirical attention has been given to the evolution of child health before, upon and following parental union dissolution. Drawing on data from the Child Development Supplement and the Transition to Adulthood Supplement of the Panel Study of Income Dynamics (1997-2017), I investigate whether parental union dissolution during childhood is associated with children's Body Mass Index (BMI) and the risk of developing overweight/obesity in the short and long run (n = 2675 children aged 0-12 in 1997). The results from a combination of propensity score matching and fixed-effects linear regression models show that union dissolution is associated with increases in child BMI and an increased risk of developing overweight/obesity. These changes in children's weight status persist for at least ten years after parental separation. Unhealthy weight gains following parental separation are more pronounced among female children and those with lower-educated and non-White parents. The findings suggest that in the United States parental union dissolution contributes to increase socioeconomic inequalities in child health. Therefore, children with separated parents and lower socioeconomic backgrounds have greater risks of developing overweight/obesity and other obesity-related morbidities over their life courses.