Adolescents who experience social anxiety often display distressing fears that unfamiliar individuals evaluate their performance in social settings. These fears typically manifest as fears of negative evaluation (FNE) and/or fears of positive evaluation (FPE). Two well-established survey measures were originally developed to assess these evaluative fears in adults (Brief Fear of Negative Evaluation Scale [BFNE]; Fear of Positive Evaluation Scale [FPES]), and recent work supports their psychometric properties when administered to adolescents and their parents. Yet, do these reports relate to how adolescents behave within anxiety-provoking social interactions germane to their clinical presentations, namely their interactions with unfamiliar peers? We tested this question in a mixed clinical/community sample of 105 adolescents and their parents. Both adolescents and parents completed reports about adolescents’ fears on the BFNE and FPES. Further, trained independent observers rated adolescents’ social anxiety and social skills during the Unfamiliar Peer Paradigm, a set of tasks designed to estimate how adolescents react to social interactions with unfamiliar peers. At the bivariate level, adolescents’ BFNE and FPES reports related to observed social anxiety and social skills, whereas parents’ FPES reports related to observed social anxiety. Further, both informants’ FPES reports demonstrated incremental validity in relation to observed social anxiety, relative to the other informant. Compared to their BFNE reports, adolescents’ FPES reports displayed incremental validity in relation to observed social skills; the reverse was not true for adolescents’ BFNE reports. These findings have important implications for screening and treatment planning assessments when delivering services to adolescents experiencing social anxiety.