Purpose: This study aimed to identify nursing competencies of outpatient nurses, to develop a tool to assess the same, and to verify the validity and reliability of the tool. Method: Preliminary items of the scale were developed based on a literature review and nurses' interviews. The tool's content validity was verified by an expert panel and by conducting a pilot study. Subsequently, to verify the validity and reliability of the scale, data were collected from 233 outpatient nurses from 7 certified tertiary hospitals. Data were analyzed using exploratory factor analysis, independent t-test, and internal consistency and correlation analysis. Result: The factor analysis revealed the following 9 factors for the 48 items of the scale: organizational management, customer orientation, personality and ethics, problem solving, nursing practice, interpersonal relationships, communication, crisis management, and professionalism. The Cronbach's α coefficient of the final instrument was .97. Conclusion: The tool developed in this study exhibited adequate validity and reliability. It was effective in reflecting the changing roles of outpatient nurses. Therefore, in future, this tool is expected to help improve outpatient nurses' competencies and nursing quality.