Introduction: The Perseverative Thinking Questionnaire (PTQ) measures repetitive negative thinking and has been translated and validated in several countries. However, the PTQ has not been translated and validated in a Danish clinical sample. The aim of this paper was to evaluate the psychometric properties of the PTQ in a Danish clinical population and to introduce a shorter, 9-item version of the PTQ, the PTQ-9. Methods: Participants were recruited from a multicenter randomized clinical trial (N = 251). They all completed the PTQ and World Health Organization 5-item Wellbeing Index (WHO-5). Participants were further assessed with the 6-item Hamilton Anxiety Rating Scale (HAM-A6) and the 6-item Hamilton Depression Rating Scale (HAM-D6). Results: The 2nd order factor model with one higher order general factor and three lower order factors showed the best model fit for the PTQ and the PTQ-9. Both versions showed good internal consistency and the expected correlations with the constructs used for validation. Furthermore, using WHO-5 as primary outcome, both the PTQ and PTQ-9 versions were able to discriminate between treatment responders and nonresponders. Conclusion: The PTQ and the PTQ-9 showed satisfying psychometric properties in a Danish clinical sample, including sensitivity to change, and could be used to evaluate psychotherapeutic treatment. To minimize the burden for the patients, the PTQ-9 may be recommended for clinical use over the PTQ.