Background There is a lack of good-quality instruments measuring stigma experienced by family members of stigmatised people. Aims To develop a self-report measure of stigma among families of people with intellectual and developmental disabilities and examine associations between family stigma and other variables. Method The new Family Stigma Instrument (FAMSI) was tested with 407 family carers, 53% of whose offspring had an autism spectrum disorder in addition to intellectual disability. They also completed measures of subjective well-being, caregiver burden, self-esteem and social support. Results The FAMSI yielded a five-factor structure and had good reliability. Perceived family stigma, caregiver burden and subjective well-being were the strongest predictors of family stigma. Conclusions This instrument can advance our understanding of the impact of stigma on family members. It can also help us understand sociodemographic, psychosocial and contextual variables of both the carer and cared for person that may influence family members' experiences.