Background: Affiliate stigma refers to the extent of self-stigmatization among associates of the targeted minorities. Given previous studies on caregiver stigma were mostly qualitative in nature, a conceptually based, unified, quantitative instrument to measure affiliate stigma is still lacking. Materials and Methods: Two hundred and ten caregivers of people with intellectual disability (CPID) and 108 caregivers of people with mental illness (CPMI) were recruited to validate the 22-item Affiliate Stigma Scale with caregiving stress, subjective burden and positive perceptions. Results: Exploratory factor analysis suggested that the scale was unidimensional, with excellent internal consistencies for both CPID and CPMI. It showed good predictive validity on subjective burden, after the extent of caregiving involvement and caregiving stress were controlled for. Conclusion: Implications on psychoeducation and stigma reduction were discussed. The newly developed scale provided a quantitative instrument with which affiliate stigma can be compared across different conditions.