Research supports the notion that adolescents' mental health is impacted by peers via contagion processes. A growing area of interest has been how co-rumination may influence depressive symptoms within friendships. The current study examined particular conditions under which co-rumination is especially likely to facilitate depression contagion. Participants were adolescents (N = 480, 49% female, M age = 14.6 years, 59.5% European American) paired in friendship dyads and assessed over 9 months. Characteristics of the adolescent (personal distress), of the friend (excessive reassurance seeking), and of the friendship (friendship quality) were considered. Moderated mediation analyses indicated that co-rumination facilitated depression contagion only under conditions of adolescents' high personal distress, friends' high excessive reassurance seeking, and high positive friendship quality. This research underscores the importance of attending to how and under what conditions depression contagion occurs within friendships in order to support adolescents' positive social and emotional development.