This study utilizes nationally representative, longitudinal data from the Mexico Family Life Survey to examine the associations between union transitions and changes in body mass index (BMI) among adults in Mexico. Results from change score regression models (N = 11,339) indicate larger BMI gains for those entering a union than for those remaining single, net of baseline weight status and socioeconomic controls. Further, a significant moderating effect of baseline weight status suggests that overweight individuals entering a union gain almost two BMI points more than overweight single individuals during this three-year period. Individuals experiencing a union dissolution gain less BMI than those entering a union, but are predicted to lose BMI (as found in the United States) only if they are overweight before the transition.