Introduction: As to the significance of clinical competency, the competency concept should be up-to-date periodically until clinical competency is evaluated based on it. This study aimed to develop and evaluate psychometric properties of a new tool to measure the postgraduate psychiatric nursing students’ competencies.Methods: The current study was conducted with a sequential exploratory mixed-method, in Iran, in 2019 -2022. The qualitative part was conventional content analysis, and the quantitative part was a methodology study. The questionnaire was developed by the item generation via individual semi-structured interviews with 21 participants sampled purposefully, and a literature systematic review. In the quantitative phase, psychometric analysis was performed based onconsensus-based standards for the selection of health status measurement instruments (COSMIN) criteria, and using face, content, and construct (i.e., convergent, known group, and exploratory factor analysis done on 199 postgraduate psychiatric nursing students by available sampling) validity. t-test analysis was used to compare the clinical competenceratings of two groups of experienced and inexperienced postgraduate psychiatric nursing students. Additionally, dependability was examined for internal consistency, stability over a one-month period, and measurement error. The sampling technique used for content validity was deliberate. Then, the responsiveness (through minimally detectablechanges), and interpretability (through minimal important changes) were calculated.Results: The questionnaire consisted of 43 items. Construct validity assessment via exploratory factor analysis (EFA) showed that 67.53% of the cumulative variance was explained by two factors: “Education and nursing care” (23 items) and “Evidence based psychiatric nursing interventions” (20 items). The convergent validity with one golden standard instrument was 0.49. The difference of the clinical competency scores of the two groups of experienced and novice was significant (P0.05).Conclusion: The 43-item postgraduate psychiatric nursing students’ clinical competency questionnaire is a valid and reliable newly developed instrument. Further studies are recommended to be conducted to assess competency with the largest sample size to promote instrument
