The high rate of comorbidity among mental disorders has driven a search for factors associated with the development of multiple types of psychopathology, referred to as transdiagnostic factors. Rumination is involved in the etiology and maintenance of major depression, and recent evidence implicates rumi- nation in the development of anxiety. The extent to which rumination is a transdiagnostic factor that accounts for the co-occurrence of symptoms of depression and anxiety, however, has not previously been examined. We investigated whether rumination explained the concurrent and prospective associations between symptoms of depression and anxiety in two longitudinal studies: one of adolescents (N 1⁄4 1065) and one of adults (N 1⁄4 1317). Rumination was a full mediator of the concurrent association between symptoms of depression and anxiety in adolescents (z 1⁄4 6.7, p < .001) and was a partial mediator of this association in adults (z 1⁄4 5.6, p < .001). In prospective analyses in the adolescent sample, baseline depressive symptoms predicted increases in anxiety, and rumination fully mediated this association (z 1⁄4 5.26, p < .001). In adults, baseline depression predicted increases in anxiety and baseline anxiety predicted increases in depression; rumination fully mediated both of these associations (z 1⁄4 2.35,p 1⁄4 .019 and z 1⁄4 5.10, p < .001, respectively). These findings highlight the importance of targeting rumination in transdiagnostic treatment approaches for emotional disorders.