Background: Internet addiction (IA) is maladaptive pattern of internet use leading to clinically significant impairment or distress. It has a varied prevalence between countries and states due to varied criteria and sample studied. Objectives: The objectives are to study the magnitude of IA and its impact on mental health among medical students of a selected University in Mangalore, Karnataka, and also to assess the association of various sociodemographic variables with IA. Materials and Methods: A cross-sectional study was conducted among all the M.B.B.S students residing in the hostel of a selected university located in Mangalore, Karnataka, for 1 month. Sociodemographic profile of the study participants and information on IA and mental health were collected using pre-designed and pre-tested questionnaire. Results: Mean age of the study participants was 20.62 ± 1.463 and 32.9% of the internet users were using internet for ≥38 h per week. With Young’s IA (YIAT) test, internet users were divided into 3 categories, namely, 46.2%, 32.9%, and 4.4% as average users, possible addicts, and internet addicts, respectively. There was a statistically significant difference in mean scores of severe depression subscale of General Health Questionnaire - Twenty Eight across two groups of YIAT test (t = 0.448; P = 0.015). Marital status and weekly hours of internet use were the statistically significant predictors of IA (P < 0.05). Conclusion: Prevalence of IA was higher in contrast to other studies, and its impact on mental health warrants stringent efforts for higher studies, early diagnosis, and prevention.