To evaluate the relationships between problematic internet use (PIU) and psychiatric comorbid disorders and internet use habits in a clinical sample of adolescents with attention-deficit/hyperactivity disorder (ADHD). This cross-sectional study included 95 adolescents with ADHD. Problematic behaviors and symptoms related to internet use were evaluated via Young's Internet Addiction Scale (YIAS), and subjects with a YIAS score of ≥50 were categorized as PIU while those with a score of <50 were defined as normal internet use (NIU). The two groups were compared with respect to demographics and psychometric tests. While psychiatric disorders were examined by a semistructured instrument, self-report and parent-report scales were used to assess other individual and clinical characteristics of participants. 33.7% (n = 32) of the participants were determined to have PIU. There was no gender (p = .058) or age (p = .426) difference between the PIU and NIU groups. Current presence of social phobia (p = .035) and history of major depressive disorder (p = .006) were more frequent in the PIU group than the NIU group. Multivariable regression analysis revealed that PIU was independently associated with online gaming (OR: 2.375, 95% CI: 1.532-3.681), e-mail use (OR: 1.864, 95% CI: 1.170-2.971), social networking (OR: 1.834, 95% CI: 1.156-2.910), and Social Phobia Scale for Children and Adolescents (SPSCA) score (OR: 1.058, 95% CI: 1.020-1.098). PIU may be common among adolescents with ADHD. The severity of social phobia and particular online activities (playing online games, e-mailing, social networking) may be associated with a higher risk of PIU in adolescents with ADHD. © 2020 Association for Child and Adolescent Mental Health.