This study aimed to estimate overreporting (the false positive) and underreporting (false negative) rates in self-reported IGD assessment compared with clinical diagnosed IGD. The study population consisted of 45 with IGD and 228 without IGD based on clinical diagnosis from the Internet User Cohort for Unbiased Recognition of Gaming Disorder in Early Adolescence (iCURE) study. All participants completed self-reported IGD assessments. Clinical interviews were conducted blindly by trained mental health professionals based on DSM-5 IGD criteria. Self-assessed average daily amount of gaming time and game genre were measured. Psychological characteristics, including anxiety, suicidality, aggression, self-control, self-esteem, and family support, were obtained from the baseline survey. The false-negative rate for self-reported IGD assessment was 44%. The false-negative group reported less time playing online games than the IGD group, though their psychological characteristics were similar to those of the IGD group. The false-positive rate was 9.6%. They reported more time playing online games than non-IGD group, though their psychological characteristics were similar to those of non-IGD group except self-control. The discrepancy of IGD diagnoses between self-reports and clinical diagnosis revealed limitations of self-measurements. Various strategies are required to overcome the methodological shortfalls of self-reports for the assessment of IGD.