Background: Most studies on substance use among psychiatric patients focus on patients within specific diagnostic categories. Less is known about substance use among the general psychiatric patient population. The aim of this study was to investigate the clinical and demographic variation in the prevalence of substance use among psychiatric in-patients and outpatients in Norway.Methods: A national census in psychiatric institutions and outpatient clinics was conducted. Data were returned for 2,358 in-patients (response rate, 65%) and 23,167 outpatients (response rate, 60%). Substance use was measured based on substance use disorder diagnosis or reported substance use in the last four weeks (alcohol 2–4 days a week or illicit drug use). Regression analyses controlling for demographic and socio-demographic characteristics were carried out.Results: Substance use was identified in 32.4% of in-patients and 13.9% of outpatients. The most frequently reported substances used were alcohol, sedatives and cannabis. Among in-patients, the prevalence of substance use was highest in patients with schizophrenia, personality disorders and anxiety disorders. Among outpatients, the prevalence was highest in patients with schizophrenia and other psychoses. In-patients with anxiety disorders and outpatients with schizophrenia and other psychoses had a significantly higher risk of substance use than other patients. In both samples, the prevalence of substance use was higher among males, 24–29-year-olds and the most socially deprived.Conclusions: This study provides further knowledge about patients at risk for co-morbid substance abuse and poor treatment outcomes. Clinicians may consider targeting patients with severe mental illness, young males and those who are socially deprived in efforts to prevent emerging substance abuse and improve outcomes.