This study examines the relationship between urbanization and Internet addiction (IA) and association with other psychopathology and social support, in a nationally representative sample in Japan. Data from the World Mental Health Japan Second Survey were used. There were 2450 survey respondents, with an average response rate of 43.4%. Respondents’ living areas were divided into three groups on the basis of urbanization (operationalized as city size). IA was measured using the Compulsive Internet Use Scale (CIUS). Three variables — psychological distress, past-12-month mental disorder, and social support — were measured using established instruments. A multilevel model was conducted to clarify the association between urbanization and IA (continuous scores and prevalence), before and after adjusting for possible individual-level and area-level variables and demographic variables. CIUS scores were significantly higher in large cities than in small municipalities before adjusting for psychological distress, social support, and past-12-month mental disorder. After adjustment, these associations attenuated substantially: urbanization was no longer significantly associated with odds of mild/severe IA, while the relationship held for continuous CIUS scores. Thus, residence in large cities is associated with higher odds of IA in Japan; psychological distress, social support, and past-12-month mental disorder partly explain this association.