Objective: The Schedule for Affective Disorders and Schizophrenia for School-Age Children-Present and Lifetime version (K-SADS-PL) is a widely used semi-structured diagnostic interview in child and adolescent psychiatry. However, given the extensive use of the K-SADS-PL in clinical practice and research and its adaptation for use in many languages and cultures, validation studies of the instrument are scarce. This study was designed to examine the inter-rater reliability, criterion validity and construct validity of the updated instrument, the K-SADS-PL for DSM-5, in Japanese outpatients totaling 95 children and adolescents. Method: We translated and adapted the updated instrument into Japanese using a standard forward-backward translation procedure. Two of nine experienced clinicians independently made diagnoses using the interview for each patient in a conjoint session. Discrepancies in diagnosis between two clinicians were resolved by consensus, and the consensus diagnosis was compared with a “best-estimate” diagnosis made by five experienced clinicians using all available data sources for patients who were blinded to the diagnosis using the K-SADS-PL for DSM-5. The “best-estimate” diagnosis of ASD was also based on the Diagnostic Interview for Social and Communication Disorders. Results: The inter-rater reliability was very good, as shown by κ ≥ 0.8 for all disorders examined: autism spectrum disorder (ASD), attention-deficit hyperactivity disorder, tic disorders, selective mutism, enuresis and encopresis. The criterion validity was good, as shown by κ ≥ 0.6 for all disorders examined, except for ASD (κ = 0.59). This study also revealed good construct validity of the instrument by confirming the expected associations with each scale from the Social Responsiveness Scale-2nd edition and the Strengths and Difficulties Questionnaire. Conclusion: These results suggest that the K-SADS-PL for DSM-5 generates valid diagnoses in child and adolescent psychiatry. Keywords: Child and adolescent psychiatry, Semi-structured interview, Reliability, Validity