Cross-Cultural Adaptation and Psychometric Evaluation of the European Portuguese PHQ-9 Modified for Teens in a Large School-Based Adolescent Sample

Background/Objectives: Depressive symptoms are a major public health concern during adolescence, and brief, psychometrically sound, culturally adapted instruments are needed to support early identification in school settings. Although the PHQ-9 has previously been validated in European Portuguese adult samples, adolescent-specific evidence for the PHQ-9 Modified for Teens (PHQ-9_T) remains limited. This study aimed to cross-culturally adapt the PHQ-9_T for European Portuguese adolescents and evaluate its structural validity, internal consistency, construct validity, and measurement invariance across sex in a school-based sample. Methods: A cross-sectional psychometric study was conducted among 1089 students aged 14–19 years attending Portuguese public secondary schools during the 2024/2025 academic year. Following a five-stage cross-cultural adaptation process, the sample was randomly divided into exploratory (n = 554) and confirmatory (n = 535) subsamples. Exploratory factor analysis used polychoric correlations and unweighted least squares extraction, whereas confirmatory factor analysis used WLSMV. Internal consistency, hypothesis testing for construct validity using the GAD-7 and CD-Quest-T, and measurement invariance across sex assigned at birth were also examined. Results: Exploratory factor analysis supported a unidimensional structure explaining 59.30% of the variance (factor loadings 0.717–0.824). Confirmatory factor analysis provided overall support for the one-factor model (CFI = 0.985; TLI = 0.981; RMSEA = 0.082, 90% CI [0.074, 0.090]; SRMR = 0.036). Internal consistency was high (α = 0.911; ω = 0.910). PHQ-9_T scores showed a strong positive correlation with GAD-7 (ρ= 0.787) and a moderate positive correlation with CD-Quest-T (ρ = 0.535) scores. Configural, metric, and scalar measurement invariance across sex was supported. Among participants with complete PHQ-9_T scores (n = 1070), 41.3% scored within the moderate-or-higher symptom-severity range. Conclusions: The European Portuguese PHQ-9_T demonstrated evidence supporting structural validity, internal consistency, construct validity, and measurement invariance across sex in this school-based adolescent sample, supporting its use as a brief measure of depressive symptom severity within structured, multiprofessional school and community mental health pathways, rather than as a standalone diagnostic tool.