Objective: Individuals with internet gaming disorder (IGD) or gaming disorder (GD) had a higher prevalence of ADHD than individuals without IGD or GD. The overpathologizing of excessive gaming behaviors may be associated with the functional impairment due to ADHD in individuals with GD. Methods: The present study was designed to compare the gaming patterns, accompanying psychopathology, and co-occurring psychiatric disorders, especially ADHD, between clinical and general gamer samples (17–29 years old). Results: Out of the nine IGD criteria, the item “functional impairment” had low diagnostic accuracy in participants with IGD or GD. The IGD (GD) group had a higher prevalence of ADHD compared to the general gamer group. Conversely, there was a higher prevalence of IGD (GD) in the ADHD group compared to the non-ADHD group. Discussion: The possibility of ADHD should be considered when IGD or GD is diagnosed in patients with problematic Internet gaming.