Best friend expected emotion socialization responses were examined as a potential explanation for the link between social anxiety and youths’ friendship quality and dysfunctional emotion regulation (ER). A community sample of 202 young adolescents (X{macron}_age = 12.66; 52.5% girls, 75.7% White) within 101 same-sex, reciprocated best friend dyads completed measures of social anxiety, friendship quality, dysfunctional ER, and how they expected their friend to respond to their negative emotions. Social anxiety was related to lower expectations of support (i.e., reward, override) responses from friends, and for boys, to greater expectations of unsupportive (i.e., neglect, aggression) responses from friends. Lower expected support responses mediated the relation between social anxiety and socioemotional functioning for girls. For boys, greater expectations of unsupportive responses mediated the relation between social anxiety and socioemotional functioning. Findings indicate that social anxiety may disrupt emotion socialization processes within adolescent friendships with implications for youth socioemotional functioning.