["Journal of Research on Adolescence, Volume 36, Issue 2, June 2026. ", "\nAbstract\nBehavioral inhibition (BI) is one of the most reliable predictors of social anxiety, and both peers and family members play a key role in shaping this association. In a sample of adolescents enriched for elevated BI and social anxiety symptoms, we examined (1) the moderating role of familial and close peer relationship qualities on associations between BI, concurrent social anxiety symptoms, and social anxiety symptom trajectories and (2) indirect effects of adolescent BI on social anxiety through close peer and familial relationship qualities. Furthermore, we examined sex differences in these pathways. When adolescents were 12–14 years old (N = 150; 60% female at birth; 80% White; 38% annual family income <$70,000), parents reported on BI and youth self‐reported familial and close peer relationship qualities. In addition, adolescents self‐reported social anxiety symptoms annually across 4 years. Multiple‐group growth curve models indicated that, for adolescent girls, perceived support from close peers interacted with BI to predict both concurrent social anxiety and social anxiety symptom trajectories. For adolescent boys, perceived familial negative interactions interacted with BI to predict social anxiety symptom trajectories. Finally, for adolescent girls, BI indirectly predicted social anxiety through low perceived support from family members, but not through peer relationship qualities. Results provide support for social risk and protective factors that shape anxiety trajectories across this vulnerable developmental period for BI youth.\n"]