Little is known about whether a widely used mindfulness measure in adults--the Five Facet Mindfulness Questionnaire (FFMQ)--is also reliable and valid in adolescents. The current study evaluated the psychometric properties of a 20-item short-form FFMQ in a sample of 599 high school students (M[subscript age] = 16.3 years; 49% female) living in the U.S. Students completed the FFMQ and a battery of self-report questionnaires assessing aspects of psychological well-being and social skills 3 times over the course of one academic year. Confirmatory factor analysis indicated that a modified four-factor hierarchical model (excluding the Observe subscale and 1 item from the Describe subscale) best fit the data. This four-factor, hierarchical FFMQ demonstrated evidence of measurement invariance across time, gender, and grade level. Reliabilities for the FFMQ total score and its subscales ranged from 0.61 to 0.88. The FFMQ total score, and its subscales (excluding Observe), demonstrated evidence of convergent (e.g., with self-compassion) and discriminant (e.g., with social perspective taking skills) validity. Finally, the FFMQ total score and Act with Awareness, Nonjudgment, and Nonreactivity subscales demonstrated evidence of incremental predictive validity for cross-time changes in psychological well-being outcomes (e.g., perceived stress). Overall, results provide preliminary support for the reliability and validity of a short-form FFMQ for use in high-school-age adolescents.