Adolescence marks the emergence of sex differences in internalizing symptoms and disorders, with girls at increased risk for depression and anxiety during the pubertal transition. However, the mechanisms through which puberty confers risk for internalizing psychopathology for girls, but not boys, remain unclear. We examined two pubertal indicators (pubertal status and timing) as predictors of the development of emotion regulation styles (rumination and emotional clarity) and depressive and anxiety symptoms and disorders in a three-wave study of 314 adolescents. Path analyses indicated that early pubertal timing, but not pubertal status, predicted increased rumination, but not decreased emotional clarity, in adolescent girls, but not boys. Additionally, rumination mediated the association between early pubertal timing and increased depressive, but not anxiety, symptoms and disorder onset among adolescent girls. These findings suggest that the sex difference in depression may result partly from early maturing girls’ greater tendency to develop ruminative styles than boys.