Background: Adolescence is a key time period for the emergence of psychosocial and mental healthdifficulties. To promote adolescent adaptive (‘resilient’) psychosocial functioning, appropriate conceptualizationand quantification of such functioning and its predictors is a crucial first step. Here,we quantify resilient functioning as the degree to which an individual functions better or worse thanexpected given their self-reported childhood family experiences, and relate this to adolescent familyand friendship support.Method: We used Principal Component and regression analyses to investigate the relationship betweenchildhood family experiences and psychosocial functioning (PSF: psychiatric symptomatology,personality traits and mental wellbeing) in healthy adolescents (the Neuroscience in Psychiatry Network;N=2389; ages 14-24). Residuals from the relation between childhood family experiences andPSF reflect resilient functioning; the degree to which an individual is functioning better, or worse,than expected given their childhood family experiences. Next, we relate family and friendship supportwith resilient functioning both cross-sectionally and one year later.Results: Friendship and family support were positive predictors of immediate resilient psychosocialfunctioning, with friendship support being the strongest predictor. However, whereas friendshipsupport was a significant positive predictor of later resilient functioning, family support had a negativerelationship with later resilient psychosocial functioning.Conclusions: We show that friendship support, but not family support, is an important positive predictorof both immediate and later resilient psychosocial functioning in adolescence and early adulthood.Interventions that promote the skills needed to acquire and sustain adolescent friendshipsmay be crucial in increasing adolescent resilient psychosocial functioning.