Background: During the past several years, there has been a growing interest in the negative effects that providing therapy may have on therapists. Of special interest is a phenomenon called secondary traumatization, which can arise while working with traumatized clients. To develop a simple screening tool for secondary traumatization, a quantitative assessment instrument was constructed using a data-driven approach based on qualitative interviews with affected trauma therapists as well as experienced supervisors in trauma therapy. Objective: The aim of the current study was to analyze the psychometric properties of the newly developed Questionnaire for Secondary Traumatization (FST) acute and lifetime version and to determine the most appropriate scoring procedure. Method: To this end, three independent samples of psychotherapists (n=371), trauma therapists in training (n=80), and refugee counselors (n=197) filled out an online questionnaire battery. Data structure was analyzed using factor analyses, cluster analyses, and reliability analyses. Results: Factor analyses yielded a six-factor structure for both the acute and the lifetime version with only a small number of items loading on differing factors. Cluster analyses suggested a single scale structure of the questionnaire. The FST total score showed good internal consistencies across all three samples, while internal consistency of the six extracted factors was mixed. Conclusion: With the FST, a reliable screening instrument for acute and lifetime secondary traumatization is now available which is free of charge and yields a sum score for quick evaluation. The six-factor structure needs to be verified with confirmatory factor analyses.