Addictive Internet use has recently been proposed to be included in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition. Still, little is known about its nosological features, including comorbidity with other mental disorders and disorder-specific psychopathological symptoms. To investigate whether Internet addiction (IA) is an issue in patients in addiction treatment, 1826 clients were surveyed in 15 inpatient rehabilitation centers. Male patients meeting criteria for comorbid IA (n = 71) were compared with a matched control group of male patients treated for alcohol addiction without addictive Internet use (n = 58). The SCL-90-R, the Patient Health Questionnaire, and the seven-item Generalized Anxiety Disorder were used to assess associated psychiatric symptoms and further comorbid disorders. Comorbid IA was associated with higher levels of psychosocial symptoms, especially depression, obsessive-compulsive symptoms, and interpersonal sensitivity. Moreover, the patients with IA more frequently met criteria for additional mental disorders. They display higher rates of psychiatric symptoms, especially depression, and might be in need of additional therapeutic treatment. In rehabilitation centers, a regular screening for IA is recommended to identify patients with this (non–substance-related) addiction and supply them with additional disorder-specific treatment.