The study evaluated the Internalized Stigma of Mental Illness (ISMI) scale, designed to measure the subjective experience of stigma, with subscales measuring Alienation, Stereotype Endorsement, Perceived Discrimination, Social Withdrawal and Stigma Resistance. The ISMI was developed in collaboration with people with mental illnesses and contains 29 Likert items. The validation sample included 127 mental health outpatients. Results showed that the ISMI had high internal consistency and test-retest reliability. Construct validity was supported by comparisons against scales measuring related constructs with the same methodology. As expected, the ISMI had positive correlations with measures of stigma beliefs and depressive symptoms, and it had negative correlations with measures of self-esteem, empowerment and recovery orientation. Factor analyses of the joint set of items from the ISMI and each scale supported the distinction between constructs. Having a validated measure of internalized stigma may encourage clinicians to include stigma reduction as a verifiable treatment goal in addition to symptom reduction.