To analyze the psychometric properties of the Norwegian version of the Weiss Functional Impairment Rating Scale parent and self-reports (WFIRS-P and WFIRS-S) in adolescents with ADHD. 102 clinically referred patients, of which 86% were enrolled in an ongoing RCT program (Clinical trials NCT02937142), were diagnosed with ADHD according to the Diagnostic and Statistical Manual of Mental Disorders version IV (DSM-IV). The conceptual framework of the WFIRS-P and the WFIRS-S was evaluated using confirmatory factor analysis (CFA), reliability was estimated using Cronbach’s alpha, convergent and divergent validity was assessed using correlations with the Children’s Global Assessment Scale (C-GAS) and the ADHD Rating Scale-IV (ADHD-RS-IV). CFA supported the original factor structure of the questionnaires, both a first-order and a second-order model revealed acceptable model fit. Internal consistency was satisfactory across domains. The parent-adolescent agreement was moderate. The correlations between the C-GAS and the total scores of the WFIRS-P and WFIRS-S were low to moderate (r = –0.29 to −0.38). The ADHD-RS-IV correlated moderately (r = 0.49) with WFIRS-P, the correlation with WFIRS-S was weak (r = 0.28) supporting divergent validity. In multiple regression analyses, the ADHD-RS total score was the strongest predictor of the total score in both the WFIRS questionnaires, with internalizing disorder showing an additional small contribution. Age, gender and full-scale IQ gave no additional contribution in explaining the variance. The findings support the use of the Norwegian version of the WFIRS-S and the WFIRS-P in the evaluation of functional impairment in adolescents with ADHD.