For parents of children with a mental health disorder, self-stigma can negatively impact their self-esteem and empowerment. Although measures of self-stigma exist, these have not been created in consultation with parents of children with a mental health disorder. Thus, the aim of this study was to construct a new scale based on parents’ experiences and developed in partnership with parents through participatory action research (PAR). Draft items that reflect parents’ self-stigmas were drawn from qualitative research. A PAR group further developed these items for conceptual and experiential representativeness, and wording suitability and interpretability. With data from 424 parents of children with a mental health disorder, factor analyses indicated three factors: self-blame, self-shame, and bad-parent self-beliefs. These factors were negatively correlated with self-esteem and empowerment. Internal consistencies were acceptable. In sum, parent self-stigma is best operationalised as including self-blame, self-shame, and bad-parent self-beliefs. A valid, PAR-informed measure is provided to promote consistent, authentic, and sensitive measurement of these components.