--- - |2 Abstract Numerous studies have reported that children and adolescents who are overweight are more likely to get bullied, yet the literature is replete with methodological limitations. We examined the transactional associations between peer victimization and body mass index (BMI), considering potential mediating (body dissatisfaction) and moderating (biological sex) factors. Participants (n = 631) came from the McMaster Teen Study, where students were assessed annually between Grades 5–11, approximately half were girls (53.9%), and the majority were white (76.4%). Peer victimization (from Grade 5) and body dissatisfaction (from Grade 6) were self‐reported by students, while parents reported their child's height and weight (from Grade 5). Cascade models were built up sequentially using path analysis across 2‐year increments (Grades 5, 7, 9, and 11). The final model had excellent fit to the data (χ2 = 73.961, df = 66, p = 0.234). Grade 5 peer victimization had a direct effect on BMI across a 2‐year period in girls (b = 0.59, SE = 0.21, p = 0.005) and boys (b = 0.82, SE = 0.30, p = 0.006), and an indirect effect on BMI via body dissatisfaction across a 4‐year period (b = 0.074, 95% CI = 0.012–0.152, p = 0.036). At no point did BMI directly increase risk for peer victimization, yet there were indirect effects via body dissatisfaction among girls but not boys. Peer victimization and body dissatisfaction were proximally and longitudinally related at every time point and there was a transactional association in late‐adolescence among girls but not boys. Targeting modifiable factors in the social (peer victimization) and psychological (body dissatisfaction) domains may limit accelerated weight gain and the health risks associated with excess adiposity. - Developmental Science, EarlyView.