Background: Dementia is prevalent among older adults and frequently causes dependence on family caregivers. Caregivers may experience a form of stigmatization called affiliate stigma that negatively affects their mental health. The current study sought to establish the psychometric properties of a tool to measure affiliate stigma among Iranian caregivers. Methods: Overall, 541 caregivers of older people with dementia were included in this cross sectional study. Several measures were used to assess the psychometric properties of the Affiliate Stigma Scale (ASS) including the Zarit Burden Interview (ZBI), Hospital Anxiety and Depression Scale (HADS), Short Form 12 (SF-12), Rosenberg Self-Esteem Scale (RSES), and Multidimensional Scale of Perceived Social Support (MSPSS). Convergent and discriminate validity were examined.Exploratory and confirmatory factor analyses were utilized to assess the factor structure of the Ass and a Rasch model was used to evaluate the measurement functioning of the scale. Results: Factor loadings ranged from 0.69 to 0.83 and test-retest reliability from 0.72 to 0.89.Item difficulty ranged widely from -0.66 to 0.89. No considerable differential item functioning (DIF) was found across gender. Confirmatory factor analysis confirmed the three cognitive,effective, and behavioral dimensions of the scale (comparative fit index [CFI]=0.931 to 0.995,root mean square error of approximation [RMSEA]=0.046 to 0.068). Internal consistency was acceptable (Cronbach's α: 0.88 to 0.94). Significant and positive relationships were found between affiliate stigma and depression, anxiety, and care giving burden (β =0.35 to 0.46). Conclusion: The ASS is a psychometrically valid measure for assessing affiliate stigma in Iranian caregivers of people with dementia. Application of this tool among other caregivers, language sand cultures deserves further study.