Background/Objectives: Obesity has been reported as an attention-deficit hyperactivity disorder (ADHD) comorbidity. So far, few studies have aimed to explore the potential causal relationship between ADHD and obesity, as well as used other measures of body composition like fat-free mass (FFM) and fat mass (FM) as measures of obesity. This study aimed to test the association between ADHD and body composition (body mass index [BMI] and others) and to evaluate the potential causal relationship with obesity. Subjects/Methods: Data from the 1993 Pelotas (Brazil) birth cohort at age 11-, 15-, 18-, and 22-year follow-up was used. We performed a cross-lagged panel model (CLPM) analysis between ADHD symptoms and BMI to explore the causal relationship between both traits. Finally, we tested whether ADHD, inattention, and hyperactivity symptom scales were associated with BMI, FM, and FFM at 22 years. Results: In the CLPM, higher ADHD scores at age 11 predicted higher BMI at age 15 (β = 0.055, 95% CI [0.037; 0.073]). ADHD symptoms at age 11 was also associated with a decrease in the FFM (β = −0.16, 95% CI [−0.28; −0.05]), and an increase in the BMI (β = 0.17, 95% CI [0.10; 0.23]) and FM (β = 0.17, 95% CI [0.06; 0.29]) at 22 years. At 22 years of age, ADHD was associated with FFM and FM. Moreover, an increase in BMI was observed with an increase in several symptoms of ADHD in general (β = 0.06, 95% CI [0.004; 0.12]), and hyperactivity symptoms (β = 0.15, 95% CI [0.05; 0.25]). Conclusion: ADHD at 11 years predicted a higher BMI at 15 years, and body fat composition in adulthood, suggesting higher scores on ADHD symptoms in early life may be a critical point for body composition in early adulthood. The hyperactivity symptoms may play an important role in the BMI increase.