Introduction: Although previous research has suggested a positive association between child maltreatment and body mass index (BMI) over the life course, it is unclear when the relationship develops. Methods: The authors used time-varying effect models and data from a nationally representative, longitudinal, cohort study (Add Health), to test how childhood physical, sexual, and emotional abuse uniquely varied in associations with BMI from age 13-28, and whether different patterns existed for males and females. Add Health collected data from 1994-2008, and present analyses occurred in 2018. Results: 18 years was the earliest age when a relationship between maltreatment and BMI emerged for either sex. Child sexual abuse was negatively associated with BMI among males from 18.5-20 years but positively associated with BMI among females from 19-24.5 years, and childhood emotional abuse was positively associated with BMI among females from 18-28 years. Conclusions: The relationship between child maltreatment and BMI varies as a function of maltreatment type, sex, and time. Notably, associations between maltreatment and BMI did not emerge until young adulthood. Future research should investigate mechanisms by which the association between maltreatment and BMI changes over time to identify trauma-informed intervention targets for improving weight outcomes.