A valid and reliable instrument for measuring affect intensity does not exist for adolescents; such a measure may help to refine understanding of emotion among youths. The purpose of the current study was to evaluate the psychometric properties and clinical relevance of a measure of affect intensity adapted for youths. Two hundred five community adolescents and 179 adolescents from a residential treatment program were recruited; both samples completed self-report questionnaires, and the community-based sample participated in positive and negative mood-eliciting laboratory tasks. Confirmatory factor analyses comparing multiple models indicated a 3-factor structure was the best fit. This Affect Intensity and Reactivity Scale for Youth (AIR-Y) evidenced good internal consistency and test-retest reliability, and the 3 factors showed incremental predictive validity for mood ratings following emotion-elicitation procedures, even after accounting for the relevant scale on the Positive and Negative Affect Scale for Children. Evidence for the clinical relevance of the AIR-Y, particularly the Negative Intensity factor, was found. Overall, the AIR-Y appears to be a psychometrically sound measure for the assessment of affect intensity among youths.