Two studies were conducted to develop and validate the UCLA PTSD Reaction Index for DSM-5-Brief Form (RI-5-BF). Study 1 used item response theory models to derive the RI-5-BF from the UCLA PTSD Reaction Index for DSM-5 and assess its internal consistency using a sample of 486 trauma-exposed youths (mean age = 13.32 years,SD = 2.90)recruited through a practice research network. Study 2 used receiver operating characteristic analyses and diagnostic efficiency statistics to assess the discriminant-groups validity and clinical utility of the RI-5-BF in identifying children at different levels of PTSD risk using a sample of 41 treatment-seeking youths (mean age = 12.44 years,SD = 2.99).