Effective management of Hoarding Disorder (HD) must begin with assessment of the severity of hoarding symptoms and functional impairment. We sought to validate the UCLA Hoarding Severity Scale (UHSS), a semi-structured, clinician-administered rating scale that measures the severity of both the core symptoms of HD and the associated features of indecisiveness, perfectionism, task prolongation, and procrastination, which are significantly associated with the diagnosis and impairment of HD.Hoarding symptom severity was measured in 62 patients who met DSM-5 diagnostic criteria for HD and 65 normal controls, using the UHSS and the Saving Inventory-Revised (SI-R), a well validated self-report measure of hoarding symptoms.The UHSS showed significant internal consistency (Cronbach׳s α=.70). Principal components analysis revealed three factors that accounted for 58% of the variance: 1) associated features and functional impairment, 2) clutter volume and social impairment, and 3) difficulty discarding, urges to save, and excessive acquisition. UHSS and SI-R scores were significantly correlated. UHSS and SI-R total and factor scores of HD patients were all significantly different from those of controls.Inter-rater and test-retest reliability were not assessed. The initial version of the UHSS did not contain rater instructions, so it lacked quantifiable anchor points for ratings.The UHSS showed internal consistency, construct validity, convergent validity, and known groups discriminant validity. The UHSS validly measures the core symptoms, associated features, and functional impairment of patients with HD. Utilizing a valid clinician-administered scale will provide a more comprehensive and accurate clinical assessment of patients with HD.