Objective: To investigate the validity of a parent version of the World Health Organization Adult Attention-Deficit/Hyperactivity Disorder (ADHD) Self-Report Scale for adolescents (ASRS-AP) and the 6-question screening version (ASRS-AP-S). Methods: Adolescent psychiatric outpatients (N = 112, mean age 15 years, 40% boys) and their parents were interviewed with the Kiddie Schedule of Affective Disorders and Schizophrenia (K-SADS), and the parents reported on the ASRS-AP/ASRS-AP-S. Results: Internal consistency (Cronbach’s alpha) was 0.93 for ASRS-AP and 0.85 for ASRS-AP-S, 0.91 and 0.87 for the inattention subscale, and 0.91 and 0.72 for the hyperactivity subscale, respectively. The concurrent validity (Spearman’s correlation coefficient) between the total K-SADS ADHD symptom severity score and the sum of the score on the ASRS-AP/ASRS-AP-S was 0.75 and 0.66, respectively. Diagnostic accuracy measures for the ASRS-AP and ASRS-AP-S were 78% and 80% sensitivity, 75% and 74% specificity, 73% and 71% positive predictive value (PPV), and 81% and 82% negative predictive value (NPV), respectively. Conclusions: The ASRS-AP and ASRS-AP-S showed high internal consistency and concurrent validity in relation to total K-SADS ADHD symptom severity score. Both scales showed favourable diagnostic accuracy measures.