The Depressive Symptom Index-Suicidality Subscale (DSI-SS) is a four-item self-report measure of suicidal ideation severity widely used across research and clinical contexts. However, the psychometric properties of the English-language version of the DSI-SS have not been extensively examined within a psychiatric sample, and important properties of this scale (e.g., sensitivity to change) have yet to be examined. Within a sample of 448 adult psychiatric patients enrolled in a partial hospital program (PHP), we examined several measurement properties of the DSI-SS, including its factor structure, internal consistency, validity, and sensitivity to change, as well as the presence of differential item functioning (DIF). Confirmatory factor analysis that specified a one-factor model indicated that the DSI-SS had good model fit. DSI-SS scores demonstrated good internal consistency, ω = .90 [95% CI = .89-.91], convergent validity (rs = .52-.74), discriminant validity (rs = .12-.27), and sensitivity to change. None of the four DSI-SS items evinced statistically significant DIF across age, gender, sexual orientation, or PHP referral source (i.e., outpatient step-up vs. inpatient step-down). These findings suggest that the DSI-SS is a psychometrically sound self-report measure that can be used in real-world clinical settings and research contexts to reliably and validly assess suicidal ideation severity. (PsycInfo Database Record (c) 2021 APA, all rights reserved).