<jats:title>Abstract</jats:title><jats:sec><jats:title>Aim</jats:title><jats:p>The aim of this study was to develop a self‐assessment tool, the Role Performance Scale for Middle‐aged Generalist Nurses in Japan and confirm its reliability and validity.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>Scale items were extracted from interviews, and item and scale‐level validity were rated by nursing management researchers and middle‐aged generalist nurses. The resulting 36‐item questionnaire was administered to clinical generalist nurses (middle‐aged nurses, <jats:italic>n</jats:italic> = 837; young nurses, <jats:italic>n</jats:italic> = 800) in Japan. Exploratory and confirmatory factor analyses were performed, and the tool's internal consistency and construct validity were analyzed.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>Valid responses obtained from 504 middle‐aged and 311 young generalist nurses were included in the analysis. The final scale was composed of 25 items, which were divided into five factors: “backing up head nurses,” “instructing young nurses on practices as an informal mentor,” “providing young nurses with mental support,” “providing empathic support to patients and their families,” and “coordinating team medical care.” In the confirmatory factor analysis, the indices of fitness supported these results. The Cronbach's alpha coefficient was .94 for the total scale and ranged from .72 to .91 for the five factors. The five factors explained a cumulative variance of 65.4%.</jats:p></jats:sec><jats:sec><jats:title>Conclusions</jats:title><jats:p>The five‐factor, 25‐item Role Performance Scale for Middle‐aged Generalist Nurses in Japan was confirmed to have sufficient reliability and validity. Middle‐aged generalist nurses can use this tool to self‐assess their nursing behavior. In addition, head nurses can use it to assess the role performance of middle‐aged generalist nurses and aid in their continual education.</jats:p></jats:sec>