This study examined the psychometric properties of the Portuguese version of the ISMI scale in a sample of 253 adult psychiatric outpatients. The exploratory factor analysis replicated the five factor structure of the scale. The results revealed good internal consistency. Criterion-related validity supported the variability in response to stigma across clinical diagnoses. Thus, the Portuguese version of the ISMI scale can be considered appropriate to measure and differentiate between stigmatization experiences. Also, in order to design more focused approaches aimed to reduce the negative effects of internalized stigma, its use in institutional and in community-based mental health services is recommended.