Background Clinical reasoning in patient care is a skill that cannot be observed directly. So far, no reliable, valid instrument exists for the assessment of nursing students’ clinical reasoning skills in hospital practice. Lasater's clinical judgment rubric (LCJR), based on Tanner's model “Thinking like a nurse” has been tested, mainly in academic simulation settings. Objectives The aim is to develop a Dutch version of the LCJR (D-LCJR) and to test its psychometric properties when used in a hospital traineeship context. Design A mixed-model approach was used to develop and to validate the instrument. Setting Ten dedicated educational units in a university hospital. Participants A well-mixed group of 52 nursing students, nurse coaches and nurse educators. Methods A Delphi panel developed the D-LCJR. Students’ clinical reasoning skills were assessed “live” by nurse coaches, nurse educators and students who rated themselves. The psychometric properties tested during the assessment process are reliability, reproducibility, content validity and construct validity by testing two hypothesis: 1) a positive correlation between assessed and self-reported sum scores (convergent validity) and 2) a linear relation between experience and sum score (clinical validity). Results The obtained D-LCJR was found to be internally consistent, Cronbach's alpha 0.93. The rubric is also reproducible with intraclass correlations between 0.69 and 0.78. Experts judged it to be content valid. The two hypothesis were both tested significant, supporting evidence for construct validity. Conclusion The translated and modified LCJR, is a promising tool for the evaluation of nursing students’ development in clinical reasoning in hospital traineeships, by students, nurse coaches and nurse educators. More evidence on construct validity is necessary, in particular for students at the end of their hospital traineeship. Based on our research, the D-LCJR applied in hospital traineeships is a usable and reliable tool.