The aim of the present study was to evaluate relationships of Internet addiction (IA) and Internet gaming disorder (IGD) symptom severities with probable attention deficit/hyperactivity disorder (ADHD) and aggression among university students, while controlling the effects of anxiety and depressive symptoms. The study was conducted with online survey among 1509 volunteered university students in Ankara who regularly use the Internet, among whom we conducted analyses related with IA. Among these students, 987 of them, who play video games, were included in the analyses related with IGD. Correlation analyses revealed that the severities of the scale scores were mildly correlated with each other both among students who regularly use the Internet and students who play video games. Probable ADHD was associated with the severity of IA symptoms, together with depression and aggression, particularly physical aggression and hostility, in ANCOVA analyses. Similarly probable ADHD was also associated with the severity of IGD symptoms, together with depression and aggression, particularly physical aggression, anger and hostility, in ANCOVA analyses. These findings suggest that the presence of probable ADHD is related with both severity of IA and IGD symptoms, together with aggression and depression.