BACKGROUND AND OBJECTIVES: Attention-deficit/hyperactivity disorder (ADHD) and obesity are 2 frequent conditions that co-occur, which has implications for the management of both conditions. We hypothesized that ADHD symptoms predict BMI and vice versa from late childhood (10–12 years) up to early adulthood (20–22 years). METHODS: Participants were adolescents in the Netherlands (n = 2773, 52.5% male, mean age = 11 years at baseline, 5 waves up to mean age 22) from the Tracking Adolescents’ Individual Lives Survey cohort. We examined bidirectional relationship between ADHD symptoms (hyperactivity/impulsivity and inattention) and BMI using the random intercept cross-lagged panel model. Time-varying covariates were pubertal status, stimulant use, depressive symptoms, and family functioning, and socioeconomic status was a time-invariant covariate. RESULTS: We found a time-invariant association of BMI with hyperactivity and impulsivity, but not with inattention, which was slightly stronger in female adolescents (female: r = 0.102; male: r = 0.086, P < .05). No longitudinal direct effects were found between ADHD symptoms and BMI during this period. CONCLUSIONS: Over the course of adolescence, the link between ADHD and BMI is stable and is predominantly with hyperactive and impulsive symptoms rather than inattention. There was no direct effect of ADHD symptoms on BMI increase nor of BMI on enhanced ADHD symptoms during this developmental period. The findings point to a shared genetic or familial background and/or potential causal effects established already earlier in childhood, thus suggesting that intervention and prevention programs targeting overweight and obesity in children with ADHD should be implemented in early childhood.