Background: Social exclusion can be both a cause and a consequence of mental health problems. Socially inclusive practice by mental health professionals can mitigate against the stigmatizing and excluding effects of severe mental illness. Aim: To develop and test the validity of a measure of social inclusion for individuals with severe mental illness – the Social Inclusion Questionnaire User Experience (SInQUE). Method: The domains of the SInQUE were chosen to reflect the domains of social inclusion identified in the Poverty and Social Exclusion Survey. Patients with severe mental illness were recruited from rehabilitation, general and forensic psychiatric services and were asked to complete the questionnaire in an individual interview with a researcher. Results: Sixty six patients with schizophrenia and schizoaffective disorder completed the SInQUE, alongside measures of psychiatric symptoms, needs and quality of life, to assess its acceptability, and concurrent and construct (convergent and discriminant) validity. The SInQUE took 45 minutes to complete and was found to have good concurrent and discriminant validity. Convergent validity was established for two domains: social integration and productivity. Conclusion: Preliminary findings suggest that the SInQUE may be a useful tool for assessing and monitoring social inclusion in individuals with severe mental illness. It has construct and concurrent validity with measures of unmet need and quality of life in this group. Further testing of the reliability of the SInQUE on a larger population is indicated.