Purpose: This study was a prospective single-arm clinical trial aimed at assessing the efficacy and toxicity of subcutaneous interleukin (IL)-2 monotherapy in patients with metastatic renal cell carcinoma (RCC).Materials and Methods: We enrolled 26 patients with metastatic RCC in this multicenter controlled trial. The patients received subcutaneous injections of recombinant IL-2 (BMI-rh-IL2, an aldesleukin biosimilar, BMIKOREA Co., Ltd.) in 5-week cycles. In the first week, the patients received a subcutaneous IL-2 loading dose of 18×10⁶ IU once on treatment days 1–5, followed by 2 days of rest. In the following 3 weeks, they received a dose of 18×10⁶ IU via subcutaneous injection once on treatment days 1 and 2. Then, the patients received a dose of 9×10⁶ IU via subcutaneous injection once on treatment days 3, 4, and 5, followed by 2 days of rest. The primary end point was the objective response rate; the secondary end points were progressionfree survival (PFS) and safety.Results: Overall, 22 patients were included in the final per-protocol analysis. The objective response and the disease control rates were 13.64% (3 of 22), and 90.9% (20 of 22), respectively. The mean PFS was 5.55 months (95% confidence interval, 2.71–8.4). The proportion of patients who experienced a treatment-related grade 3 or 4 adverse event was 3.85% (1 of 26). There were no treatment-related deaths.Conclusions: In this study, the subcutaneous IL-2 monotherapy regimen demonstrated efficacy and safety comparable to those reported in previous studies of subcutaneous IL-2 monotherapy and was effective in Korean patients with metastatic RCC.