--- - |2 Abstract Despite the acknowledged importance of friendships in emotional development during adolescence, little research has empirically examined emotion socialization processes within friendships. Co‐rumination is one such process that may involve many emotion‐related skills due to its negative emotional focus and links to emotional distress. The current study examines whether adolescent friends’ emotional competencies (i.e., emotional awareness, emotion regulation) relate to co‐rumination. Adolescents (N = 192; 53% girls; Mage = 12.67; 76% European American, 17.7% African American) participated with a reciprocated same‐sex best friend. Adolescents reported on their own and their friends’ emotional competencies and participated in observed video‐taped problem discussion task that was coded for co‐rumination. Results indicated that indices of poor emotional competence related to greater co‐rumination for girls. For boys, stronger emotional competence related to greater co‐rumination. There were more significant links to co‐rumination from adolescents’ perceptions of their friends’ emotion regulation than self‐reports of their own emotion regulation. Results are discussed with a focus on implications for emotion socialization within the best friend context during early adolescence. - Social Development, Volume 27, Issue 4, Page 808-825, November 2018.