Self-stigma instruments investigate how people with mental illness internalize public stigma. However, information is limited for the psychometric properties of their scores, especially cross-validating scores from different instruments. Thus, we used confirmatory factor analyses (CFAs) and item-response theory (IRT) models to examine the Internalized Stigma Mental Illness (ISMI) scale and the Self-Stigma Scale–Short (SSS-S). Participants with mental illness (n = 347) completed both instruments. The CFAs that simultaneously accounted for both the instrument (ISMI and SSS-S) and the trait (Affect, Cognitive, and Behavior concepts) effects outperformed those that accounted only for the instrument effect or only the trait effect. All item scores fit the IRT model and were fit with ordered, progressing hierarchies in their step difficulties. We conclude that both instruments are feasible for measuring the self-stigma and that future research can combine the items of both.