Changes to the diagnostic criteria for posttraumatic stress disorder (PTSD) in the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM–5) create a need for valid and reliable updated assessment tools. This study examined key psychometric properties (eg, internal consistency, test–retest reliability, interrater reliability, and convergent and discriminant validity) of the PTSD Symptom Scale Interview for DSM–5 (PSSI–5), a modified version of the PSS–I (PTSD Symptom Scale)—Interview Version for the DSM–IV. Participants were 242 urban community residents, veterans, and college undergraduates, recruited from 3 study sites, who had experienced a DSM–5 Criterion A traumatic event. The PSSI–5 demonstrated good internal consistency (α=. 89) and test–retest reliability (r=. 87), as well as excellent interrater reliability for the total severity score (intraclass correlation=. 98) and interrater agreement for PTSD diagnosis (κ=. 84). The PSSI–5 also demonstrated convergent validity with 3 measures of PTSD (ie, Clinician-Administered PTSD Scale for DSM–5, Posttraumatic Diagnostic Scale for DSM–5, and PTSD Checklist—Specific Version; all rs>. 72) and discriminant validity with the Beck Depression Inventory—II and the State–Trait Anxiety Inventory—Trait scale. Receiver operating characteristic analysis yielded a cutoff score of 23 for identifying a probable PTSD diagnosis. Together, these findings indicate that the PSSI–5 is a valid and reliable instrument for assessing PTSD diagnosis and severity.