The present study examined the structure of the Five Facet Mindfulness Questionnaire (FFMQ) in adolescents and whether predictive associations between mindfulness facets and depressive symptoms are mediated by reduced rumination. A sample of 520 Spanish adolescents (mean age = 16.11, SD = 0.98, range = 13–19, 57.9 % girls) completed the FFMQ and measures of depression and rumination initially and after 4 months. In study 1, a confirmatory factor analysis supported a five-factor correlated model for the five facets, which are observing, describing, acting with awareness, non-reactivity, and non-judging. All five facets and the total FFMQ scores showed acceptable internal consistency, test-retest reliability, and convergent and discriminant validity. All the mindfulness facets other than observing correlated negatively with depressive symptoms. In study 2, a subsample of 461 adolescents was followed up to examine longitudinal associations between mindfulness facets, rumination, and depression. The results demonstrated that acting with awareness and non-reactivity predicted a reduction in depression over time, whereas observing predicted an increase in depression through the mediation of increased rumination. Thus, this research provides support for the adaptive role of acting with awareness and non-reactivity and suggests that observing might play a maladaptive role in adolescents.