In recent years, major gains toward understanding the emergence of borderline personality disorder (BPD) pathology, which is typically first noted during adolescence, have been made. Simultaneously, a profound shift has occurred in the adult personality pathology literature, in which empirical evidence rebuts the idea that personality disorders (PDs) are intractable disorders that do not develop or otherwise change over time, and therefore cannot be treated. The present study addresses a gap in our understanding of within-person change in BPD symptoms across adolescence and contributes to the limited literature on outcomes associated with adolescent BPD. Using an at-risk community sample of girls (N = 2,450), the authors used bivariate latent growth curve models to analyze the codevelopment of BPD symptoms with eight domains of psychosocial functioning (e.g., academic achievement, social skills, sexual behavior) across ages 14-17. Findings revealed moderate to strong effect sizes for the associations between BPD symptoms and every domain of psychosocial functioning, suggesting that the development of BPD was coupled with poorer outcomes across development. Controlling for depression and conduct disorder features revealed unique associations between BPD and self-perception, social skills, and sexual behavior. These results highlight the increased need for extending advancements in the adult PD literature to research on PDs in adolescence, and for greater recognition of adolescent BPD in clinical settings.