Background/Objectives: There is a relative lack of specific research on registered nurse (RN) staffing in long-term care hospitals in the Republic of Korea. This study investigated the association between RN staffing levels and inpatient outcomes in long-term care hospitals in the Republic of Korea. Methods: Nationwide data of long-term care hospitals from the Health Insurance Review and Assessment Services website were used to analyze the association between registered nurse staffing levels and 7 inpatient outcome indicators. Results: The results indicated that in long-term care hospitals with higher RN staffing levels, there was an improvement in moderate-to-severe pain, activities of daily living enhancement, lower prevalence of indwelling catheters, reduction in the incidence of pressure ulcers, improvement in existing pressure ulcers, and increased rate of return to the community. Similar results were observed in analyses conducted according to disease classification groups. Conclusions: These findings suggest that increasing RN staffing levels can enhance patient safety and improve treatment outcomes. Furthermore, this study provided important foundational data for developing policies to optimize RN staffing in long-term care hospitals in the Republic of Korea.