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Diagnostic ability is essential for expert professional practice. Several instruments have been developed to assess diagnostic skills independent of specific knowledge. One such instrument is the diagnostic thinking inventory (DTI), which is used in different settings to evaluate diagnostic performance and has shown acceptable reliability and validity. The aim of the present study was to translate and validate a German version (DTI-G). Cultural adaptation and translation were performed according to international guidelines. Internal consistency and item discrimination indexes were calculated. The factorial structure of the DTI-G, test-retest reliability and known-groups validity were tested. A total of 388 physiotherapists completed the questionnaire. The internal consistency was good for the overall score of the DTI-G (Cronbach’s a = 0.84). Exploratory factor analysis yielded a five-factor solution with 21 items that explained 55% of the total variance across items. A confirmatory principal component analysis resulted in the same five-factor structure, showing an acceptable to good overall fit of the model (CFI = 0.93; RMSEA = 0.05; SRMR = 0.06). Test-retest reliability was found to be good (intraclass correlation coefficient ICC2,1 = 0.87, p < 0.001, n = 118). The difference between participants with more than 9 years of clinical experience and those with less than 9 years of clinical experience (median split) was significant (t385 = 6.00, p < 0.001), supporting known-groups validity. The results support construct validity and indicate good test-retest reliability of the DTI-G. The DTI-G can be used to measure and develop diagnostic ability of physiotherapists in clinical practice and education. Further research is necessary to validate the questionnaire for other health professions.