Substantial evidence suggests that rumination is an important vulnerability factor for adolescent depression. Despite this, few studies have examined environmental risk factors that might lead to rumination and, subsequently, depression in adolescence. This study examined the hypothesis that an adverse family environment is a risk factor for rumination, such that the tendency to ruminate mediates the longitudinal association between a negative family environment and adolescent depressive symptoms. It also investigated adolescent gender as a moderator of the relationship between family environment and adolescent rumination. Participants were 163 mother-adolescent dyads. Adolescents provided self-reports of depressive symptoms and rumination across three waves of data collection (approximately at ages 12, 15, and 17 years). Family environment was measured via observational assessment of the frequency of positive and aggressive parenting behaviors during laboratory-based interactions completed by mother-adolescent dyads, collected during the first wave. A bootstrap analysis revealed a significant indirect effect of low levels of positive maternal behavior on adolescent depressive symptoms via adolescent rumination, suggesting that rumination might mediate the relationship between low levels of positive maternal behavior and depressive symptoms for girls. This study highlights the importance of positive parenting behaviors as a possible protective factor against the development of adolescent rumination and, subsequently, depressive symptoms. One effective preventive approach to improving adolescent mental health may be providing parents with psychoeducation concerning the importance of pleasant and affirming interactions with their children.