Interest in mindfulness-based interventions for children and adolescents is burgeoning, bringing with it the need for validated instruments to assess mindfulness in youths. The present studies were designed to validate among adolescents a measure of mindfulness previously validated for adults (e.g., Brown & Ryan, 2003), which we herein call the Mindful Attention Awareness Scale-Adolescent (MAAS-A). In 2 large samples of healthy 14- to 18-year-olds (N = 595), Study 1 supported a single-factor MAAS-A structure, along with acceptably high internal consistency, test-retest reliability, and both concurrent and incremental validity. In Study 2, with a sample of 102 psychiatric outpatient adolescents age 14-18 years, participants randomized to a mindfulness-based stress reduction intervention showed significant increases in MAAS-A scores from baseline to 3-month follow-up, relative to nonsignificant score changes among treatment-as-usual participants. Increases in MAAS-A scores among mindfulness-based stress reduction participants were significantly related to beneficial changes in numerous mental health indicators. The findings support the reliability and validity of the MAAS-A in normative and mixed psychiatric adolescent populations and suggest that the MAAS-A has utility in mindfulness intervention research.