Purpose: : Assertiveness is a crucial factor in patient safety, workplace violence, and work stress, and is associated with better interpersonal relationships in clinical settings. Despite its significance, a standardized assertiveness scale reflecting Korean healthcare culture is currently lacking. Thus, this study aimed to investigate the validity and reliability of the Korean version of the assertiveness scale for nurses (K-ASN). Methods: : The survey was conducted with 250 nurses working in secondary and tertiary hospitals in Korea. The study examined content, criterion, and construct validity using exploratory factor analysis and also assessed internal consistency reliability. Results: : The K-ASN consists of 22 items and 6 factors, namely self-denial, confrontation, dealing with criticism, positive assertion, spontaneity, and active assertion. The K-ASN demonstrated good content validity and a high criterion validity with the Rathus Assertiveness Schedule (RAS) at .75. The internal consistency reliability was also satisfactory, with a Cronbach’s alpha of .80, and subscale reliability ranging from .52 to .87. Conclusion : The K-ASN is a reliable and valid assertiveness scale for nurses in Korea. The K-ASN can be a valuable tool to be used to assess the assertiveness of healthcare professionals which is associated with improvements in interpersonal relationships, patient safety, and health-related outcomes.