Borderline Personality Disorder (BPD) significantly impacts individuals, causing substantial distress and impairing social, occupational, and other critical areas of functioning. Current tools to assess functioning in BPD are limited and often lack validation in BPD-specific populations. Furthermore, available instruments focus primarily on global functioning rather than specific areas such as cognitive functioning or interpersonal relationships. This study aims to validate the Functioning Assessment Short Test (FAST) in adolescents and young adults with BPD in a Spanish population.An observational study was conducted with 216 BPD patients and 107 healthy controls (HC) matched by age and sex. Participants were assessed using the FAST, and additional clinical measures were applied. Internal consistency, inter-rater reliability, exploratory and confirmatory factor analysis, discriminant validity through ROC analysis, and logistic regression were performed to evaluate the psychometric properties of the FAST. Additionally, sensitivity to change was assessed to determine the FAST's responsiveness to clinical improvements.The FAST demonstrated high internal consistency (Cronbach's alpha = 0.869) and a six-factor structure. Inter-rater reliability analysis indicated excellent agreement (ICC=0.997, 95 % CI=0.991-0.999). The tool showed high discriminant capacity between BPD patients and HC (AUC=0.947), with a cutoff score of >11 achieving 88 % sensitivity and 90.7 % specificity. Sensitivity to change analyses in a subgroup of 35 BPD patients revealed significant effect sizes (ES=-0.98), supporting the FAST's ability to detect functional improvements over time.The FAST is a valid and reliable instrument for assessing psychosocial functioning in adolescents and young adults with BPD.