There is evidence that poverty is related to adverse child health outcomes. Yet, evidence is lacking on how economic hardship experiences during early childhood are related to adolescent obesity, how the relationship may differ by child sex, in addition to the potential child and maternal behavioral factors that link economic hardship and adolescent obesity. The purpose of the current study was to address this gap by using longitudinal data from the Fragile Families and Child Wellbeing Study (N = 1814). The analytic sample included 50.5% girls, 20% experiencing overweight status, and 19% experiencing obesity. Majority of the adolescents were born to non-Hispanic black (49%), U.S. born (86%), married/cohabitating mothers (61%) with high school or greater level of education (75%). The economic hardship trajectory classes were determined using the latent growth mixture modeling approach and supported a 4-class trajectory model, with 5% of the adolescents in the high-increasing economic hardship trajectory class. The children in the high-increasing economic hardship class had increased odds of developing overweight/obesity in adolescence compared to those in low-stable class. This association was significantly moderated by child sex (i.e., relationship was significant for adolescent boys). Parenting stress and child snacking behaviors did not significantly mediate the association between economic hardship classes and overweight/obesity. Economic hardships that increase through early childhood need to be recognized as an obesity risk factor particularly for adolescent boys.