Fear of positive evaluation (FPE) has recently emerged as an important aspect of social anxiety, alongside fear of negative evaluation. These evaluation fears peak during adolescence, a developmental stage that is also often accompanied by difficulties in emotion regulation, thereby increasing young individuals’ vulnerability to mental disorders, such as social anxiety. We aimed to examine the longitudinal within-person associations between fears of evaluation, social anxiety, and three emotion regulation strategies (i.e., acceptance, suppression, rumination) in adolescents. Data were collected from a sample of 684 adolescents through an online survey three times over the course of 6 months and were analyzed using random intercept cross-lagged panel models. At the between-person level, FPE was linked to all three emotion regulation strategies, whereas fear of negative evaluation and social anxiety were associated with acceptance and rumination. At the within-person level, difficulties in accepting emotions predicted FPE, suppression predicted social anxiety, and social anxiety predicted rumination over time. These findings reveal complex interdependencies between emotion regulation, social anxiety, and evaluation fears, both reflecting individual differences and predicting changes within individuals, and further elucidate the developmental trajectory of social anxiety in adolescence.