Internet addiction (IA) is currently becoming a serious mental health problem. The aim of this study was to estimate the prevalence of IA among undergraduate medical students and evaluate the relationship of IA with social anxiety, impulsivity, self-esteem, and depression. The study included 392 undergraduate medical students. Evaluations were made with the sociodemographic data form, the Internet Addiction Test (IAT), the Liebowitz Social Anxiety Scale (LSAS), the Barratt Impulsivity Scale-11 (BIS-11), the Rosenberg Self-Esteem Scale (RSES), the Beck Depression Inventory (BDI), and the Beck Anxiety Inventory (BAI). The IA group had significantly higher scores on LSAS, BDI, BAI and lower scores on RSES than the control group but the BIS-11 scores were similar among groups. IAT severity was positively correlated with LSAS, BDI, and BAI and negatively with RSES. No correlation was observed between IAT severity and BIS-11. In the hierarchical linear regression analysis, the avoidance domain of social anxiety was the strongest predictor of the severity of IA. The present study suggests that undergraduate medical students with IA exhibit higher social anxiety, lower self-esteem and are more depressed than those without IA, thus, indicating that social anxiety, rather than impulsivity, seemed to play a prominent role in IA psychopathology.