Although relationships between co-rumination and depressive symptoms have often been found, little research attention has been given to mechanisms underlying this association. The current study investigated brooding rumination as a mediator of the relationship between co-rumination and depressive symptoms. Analyses were performed on data of 1549 adolescents (53.4% girls; Mage = 12.93, range 9–17) using three waves of data with 1-year intervals. Mediated and indirect effects were investigated by means of cross-lagged analyses. The results indicated that co-rumination was not predictive of depressive symptoms 2 years later. However, co-rumination did have an indirect effect on prospective depressive symptoms through brooding rumination. Additional analyses looking into the directionality of effects showed that neither brooding rumination nor depressive symptoms were predictive of relative increases in one’s tendency to co-ruminate. Multi-group analyses further showed that findings were not moderated by gender or age. The current study contributes to the growing literature on the role of interpersonal and intrapersonal affect-regulation styles in predicting depressive symptoms and suggests that passive and catastrophic problem talk with same-sex friends may get internalized into maladaptive and repetitive thinking patterns.