Objective: This study aimed to assess the prevalence of overweight and obesity in the group of children and adolescents with attention-deficit/hyperactivity disorder (ADHD) according to three different diagnostic criteria for obesity. The effect of ADHD treatment on the risk of obesity was controlled. Materials and Methods: The study group consisted of 58 subjects aged from 8 to 17 years with ADHD. Control group consisted of 62 healthy age- and sex-matched children and adolescents. Overweight and obesity were assessed according to International Obesity Task-Force (IOTF), World Health Organization (WHO) and European Childhood Obesity Group (ECOG) criteria. Results: There was a significantly higher incidence of obesity in ADHD group according to WHO (ADHD vs. Control: 17.2% vs. 3.2%, p = 0.01) and IOTF criteria (ADHD vs. Control: 10.3% vs. 1.6%, p = 0.04), but no significant difference according to ECOG criteria. There was significantly higher occurrence of overweight (20.7% vs. 12.8%, p < 0.001), obesity (10.3% vs. 3.5%, p < 0.001), and overweight and obesity (31.0% vs. 16.3%, p < 0.001) in the ADHD group compared to the Polish population. The analysis did not show a statistically significant relationship between the amount of taken psychostimulant and the occurrence of overweight and obesity. Conclusions: The prevalence of overweight and obesity in the group of ADHD children and adolescents varies depending on the choice of diagnostic criteria. At the same time, it is significantly more frequent than in the general population. We suggest that future studies should give a clear rationale for the selection of cutoff points to minimize errors that could confound data analysis and interpretation.