A 60-year-old man who developed rhabdomyolysis and severe acute kidney injury (AKI) after the fourth COVID-19 mRNA vaccination, necessitating renal replacement therapy (RRT). The patient presented to the hospital two days post-vaccination with muscle pain in both lower extremities and anuria. Diagnostic tests revealed elevated creatinine kinase (CK) levels of 160,000 IU/L and serum creatinine levels of 6.59 mg/dL, confirming AKI due to rhabdomyolysis. Intravenous therapy was ineffective, leading to the utilization of online hemodiafiltration. Following treatment, CK levels normalized, and the renal function fully recovered.