To describe the test construction procedure and evaluate the internal consistency, criterion-referenced validity, and diagnostic accuracy of the Child/Adolescent Self-Report Version of the UCLA PTSD Reaction Index for DSM-5 (RI-5) across 2 independent samples. Study 1 examined the clarity, developmental appropriateness, acceptability of individual RI-5 items, and internal consistency and criterion-referenced validity of the full test. The study 1 sample included 486 youth recruited from 2 major US cities who completed the RI-5 and a measure of depression. Study 2 evaluated the reliability and diagnostic accuracy of the RI-5 in 41 treatment-seeking youth who completed the RI-5 and a "gold standard" structured diagnostic interview, the Clinician-Administered PTSD Scale for DSM-5-Child/Adolescent Version. RI-5 total scale scores showed excellent internal consistency in the 2 samples. Study 1 provided evidence of criterion-referenced validity, in that total scale scores correlated positively with depressive symptoms. Study 2 provided evidence of diagnostic accuracy (including discriminant-groups validity). RI-5 total scores discriminated youth with from youth without PTSD as benchmarked against the structured diagnostic interview. Further, receiver operating characteristic analyses using a total score of 35 provided excellent diagnostic classification accuracy (area under the curve 0.94). The developmental appropriateness and diagnostic accuracy of the RI-5 support its utility for clinical assessment, case conceptualization, and treatment planning in different child-serving systems, including schools, juvenile justice, child welfare, and mental health.