Extant literature suggests that autism spectrum disorder (ASD) and attention-deficit hyperactivity disorder (ADHD) are risk factors for internet addiction (IA). The present cross-sectional study explored the prevalence of IA among 132 adolescents with ASD and/or ADHD in a Japanese psychiatric clinic using Young's Internet Addiction Test. The prevalence of IA among adolescents with ASD alone, with ADHD alone and with comorbid ASD and ADHD were 10.8, 12.5, and 20.0%, respectively. Our results emphasize the clinical importance of screening and intervention for IA when mental health professionals see adolescents with ASD and/or ADHD in psychiatric services.