This study aimed to examine the role activities of certified nurses (CNs) in radiation oncology nursing and relevant factors. We invited 310 CNs in radiation oncology nursing, listed on the Japanese Nursing Association website, to complete a self-administered questionnaire (online or printed). From the 310 CNs, we received 205 CNs responses, of which 199 CNs working in hospitals were analyzed (64.2% valid response rate). Among the 18 activity items (9 on practice activities, 7 on interdepartmental activities, and 2 on interorganizational activities), CNs working in departments other than radiology have significantly lower scores in 11 of the items than CNs working in radiology departments. The total activity time was significantly associated with working in radiology departments (odds ratio=9.76, 95% confidence interval=4.43 to 21.53, p<.001). Moreover, the total activity time significantly correlated with 12 activity items, including care before and during radiotherapy, multidisciplinary approach, and problem-solving for ethical issues. Our results suggest that CNs in radiation oncology nursing need to be appropriately assigned to departments that frequently provide radiation therapy and be supported to ensure adequate time allocation to role activities that fully utilize their expertise.