--- - |2+ Abstract Objective The identification of transdiagnostic risk factors and processes that explain the comorbidity between depressive symptoms and eating disorder symptoms is critical. We examined the mediating role of the frustration of adolescents’ psychological needs for autonomy, competence, and relatedness in the association between self‐critical perfectionism, depressive symptoms and eating disorder symptoms. Method A cross‐sectional study (N = 248; 58% female, mean age = 14 years) and a two‐wave longitudinal study (N = 608; 59% female; mean age = 16 years) were conducted. Results At the level of inter‐individual differences and intra‐individual change, self‐critical perfectionism was a robust predictor of both symptoms. After introducing need frustration as an underlying mechanism, the relation between self‐critical perfectionism and the two types of symptoms, as well as the relation between the symptoms themselves, decreased. Conclusions Need frustration represents a transdiagnostic vulnerability process that helps to explain why self‐critical perfectionism relates to depressive symptoms and eating disorder symptoms. - Journal of Clinical Psychology, EarlyView.