Mindfulness-based therapies have demonstrated effectiveness in many clinical contexts. Various therapies that train mindfulness skills have proliferated in recent years. There is increasing interest in mindfulness-based therapies and in incorporating instruments that measure mindfulness in order to understand its role in clinical and basic research. The Five Facets of Mindfulness Questionnaire (FFMQ) is a questionnaire for measuring mindfulness; it was derived from a factor analysis of five different questionnaires that measure a trait-like general tendency to be mindful in daily life. The objective of this study is to validate the FFMQ in a Spanish sample. The FFMQ was administered to a sample of 462 subjects ranging from 18 to 63 years (X = 27.9; SD = 9.75). The sample was composed by clinical (n = 146) and non-clinical (n = 226) subsamples. The internal reliability of the scales ranged from acceptable to very good. Convergent analysis was conducted by computing Pearson's correlations, showing high correlations. The factorial structure is the same as that proposed by Baer et al. The FFMQ proved to be an effective instrument for measuring mindfulness in clinical and non-clinical Spanish samples.