This project aimed to develop and validate a brief, practical self-report measure of acute stress disorder (ASD) reactions (including ASD diagnostic criteria) in children and adolescents. A draft measure was reviewed for content validity by an expert panel, piloted with youth, and revised for clarity and coverage of key constructs. The new 29-item Acute Stress Checklist for Children (ASC-Kids) was administered to 176 youth age 8 to 17 who had a recent injury or intensive care unit admission. The ASC-Kids demonstrated strong test-retest reliability and internal consistency, as well as concurrent and predictive validity with other traumatic stress measures. Results of exploratory factor analyses were consistent with current conceptualizations of acute traumatic stress reactions. The ASC-Kids is a promising measure of child acute stress reactions.