<p>Research is lacking on whether the coexistence of attention-deficit/hyperactivity disorder (ADHD) increases the risk for Internet addiction (IA) in children with autism spectrum disorder (ASD). This study examined the impact of ADHD coexistence on IA symptoms in children with ASD, controlling the influence of family- and school-related factors. We also analyzed male and female participants separately to examine sex differences. IA symptoms were assessed using the Internet Addiction Test in children with ASD (<i>n</i> = 71) and ASD + ADHD (<i>n</i> = 93) admitted to a child and adolescent psychiatric hospital. Multiple regression analysis was performed with IA symptoms as the outcome variable and family- and school-related factors as the potential confounders. Similarly, a subgroup analysis was conducted for each sex of the participants. Results showed no differences in self-reported IA symptoms between the ASD and ASD + ADHD groups, but females in the ASD group had higher IA symptoms than the ASD + ADHD group. Low school attendance was significantly associated with higher IA symptoms for the entire target population and for males. In addition, for females, medication and bullying victimization were associated with lower and higher IA symptoms, respectively. In males, socio-economic status (SES) was associated with higher IA symptoms. ASD + ADHD coexistence does not necessarily increase the severity of IA symptoms and may not be a combination of different neurodevelopmental disorders, ASD and ADHD. Further research should examine the impact of ASD + ADHD coexistence on IA symptoms, considering sex differences and family- and school-related factors.</p>