Our aim was to develop a clinician-rated scale assessing depersonalization severity for use in clinical trials of Depersonalization Disorder and trauma-related disorders in general. The 6-item Depersonalization Severity Scale (DSS) was administered to 63 participants with DSM-IV Depersonalization Disorder as diagnosed by the SCID-D, and its psychometric properties were examined. The sensitivity of the DSS and of the Dissociative Experiences Scale (DES) to treatment change was assessed in blinded, controlled settings. Individual items were widely distributed across the severity range. Interrater reliability was excellent and internal consistency was moderate. The DSS had high convergent and discriminant validity and was sensitive to treatment change. The DES was also sensitive to treatment change. We recommend piloting the DSS in future treatment trials of trauma-spectrum disorders.