We examined the prospective associations between two indicators of mother-youth emotional climate (i.e., maternal affect, dyadic cohesiveness) during a peer problem discussion and youth coping with peer stress following entry into middle school. We also investigated youth baseline respiratory sinus arrhythmia (RSA) as a moderator of these associations given its role in physiological stress regulation. Participants were 100 mother-youth dyads (53% boys; M[subscript age] = 11.05 years; 43% ethnic minorities). More positive maternal affect at Time 1 (spring of 5th grade) was associated with more primary and secondary control engagement coping and advice-seeking at Time 2 (fall of 6th grade); greater dyadic cohesiveness was prospectively linked with more advice-seeking only. Further, the positive association between maternal affect and primary control coping was only evident for youth who showed lower (not higher) baseline RSA. We discuss implications for jointly considering the mother-youth relational context and youth physiological regulatory capacity towards understanding coping socialization.