Background: The International Trauma Questionnaire-Child and Adolescent version (ITQ-CA) is a self-report measure that assesses posttraumatic stress disorder (PTSD) and complex PTSD (CPTSD) based on the diagnostic formulation of the 11th version of the International Classification of Diseases (ICD-11). This study aimed to provide a Chinese translation and psychometric evaluation of the ITQ-CA using a sample of mental-health service seeking adolescents in Mainland China. Methods: The ITQ-CA was translated and back-translated from English to simplified Chinese and finalized with consensus from an expert panel. Adolescents ages 12-17 were recruited via convenience sampling from an outpatient psychiatric clinic in Mainland China. Participants completed the ITQ-CA; measures of four criterion variables (depression, anxiety, stress, adverse childhood experiences); and the PTSD Checklist for DSM-5 (PCL-5). Construct validity, concurrent validity, and comparison of PTSD caseness between ICD-11 and DSM-5 measures were assessed. Results: The final sample consisted of 111 Chinese adolescents (78% female; mean age of 15.23), all diagnosed with a major depressive disorder. Confirmatory factor analysis indicated the two-factor second-order model provided optimal fit. All criterion variables were positively and significant correlated with the six ITQ-CA symptom cluster summed scores. In the present sample, 69 participants (62.16%) met symptom criteria for ICD-PTSD or CPTSD using the ITQ-CA, and 73 participants (65.77%) met caseness for DSM-5 PTSD using the PCL-5. Rates of PTSD symptom cluster endorsement and caseness deriving from both diagnostic systems were comparable. Conclusions: The Chinese ITQ-CA has acceptable psychometric properties and confers additional benefits in identifying complex presentations of trauma-related responses in younger people seeking mental health services.