This paper sought to provide the first validation of a transdiagnostic measure of repetitive negative thinking - the Perseverative Thinking Questionnaire-Child version (PTQ-C) - in young people diagnosed with anxiety and depressive disorders. Participants (N = 114) were 11- to 17-year-olds with complex and comorbid presentations seeking treatment through Child and Adolescent Mental Health Services. Confirmatory factor analyses best supported a three-factor model for the PTQ-C; however, hypotheses of both perfect and close fit were rejected, and a subsequent bifactor model suggested minimal unique variance for each subscale. Results demonstrated good internal consistency, convergent validity and divergent validity for the total score and three PTQ-S subscales: core characteristics, perceived unproductiveness, and consumed mental capacity of negative repetitive thinking. PTQ-C scores did not account for additional variance in anxiety symptoms once worry was considered, indicating that retention of a content-specific measure may be warranted in clinical samples. Findings emphasize the importance of validating clinically relevant measures which were developed with subclinical populations in samples with diagnosed mental health disorders. PRACTITIONER POINTS: Validates Perseverative Thinking Questionnaire in anxious and depressed youth. Support for convergent and divergent validity, and internal consistency. Results suggest measure is appropriate for complex and comorbid presentations.