Objectives: One of the problems that the Internet has created is a virtual addiction or addiction to the global web world causing adverse psychological behavioral effects on the users. One of the groups at higher risk for Internet addiction is teenagers, and since Internet addiction will lead to academic failure in students, evaluating it among them and its relationship with signs of mental disorders seems necessary. Therefore, the aim of this research was to study Internet addiction and its relationship with the symptoms of mental disorders. Methods: This study was descriptive and correlational design. The sample population for the present study consisted of all male and female high school students of the academic year 2013-2014 in district 5 of Tehran. Among these individuals, a sample of 400 participants was randomly selected for analysis using cluster sampling method. To collect data, Internet addiction test and Brief Symptom Inventory were used, and data were analyzed using T-Test, Pearson correlation coefficient and multiple regression analysis. Results: The results showed that there is no significant difference between prevalence of Internet addiction in male and female high school students in district 5 of Tehran. However, there is a significant positive relationship between Internet addiction variables and symptoms of mental disorders and particularly, independent variables of psychosis and anxiety from the signs of mental disorders had a profound effect on Internet addiction. Discussion: Internet addiction causes negative effects on individuals. As prevention is better than treatment and according to this study, it is necessary to take this phenomenon into consideration as a psychological problem that often involves the younger generation who are responsible for future society construction and through education in families, schools and universities, the culture of proper use of computers, in particular the Internet and its facilities should be replaced with the false methods.