We encountered a patient who developed marked renal impairment and hyperuricemia in a short period after sorafenib administration, which suggested tumor lysis syndrome (TLS). A 79-year-old woman with hepatitis C-related liver cirrhosis was found to have intermediate hepatocellular carcinoma (HCC) according to Barcelona Clinic Cancer staging classification. Pre-treatment laboratory tests showed mild renal impairment. On the 10th day of sorafenib intake, the creatinine and uric acid levels rose to 2.99 and 16.1 mg/dl, respectively, and arterial blood gas analysis showed respiratory compensated metabolic acidosis, which suggested grade I clinical TLS according to the diagnostic criteria of TLS as defined by Cairo-Bishop. TLS is a group of metabolic abnormalities that arise from the rapid breakdown of many tumor cells upon the initiation of malignant tumor chemotherapy, resulting in the release of intracellular metabolites that exceed the ability of the kidney to excrete them. TLS occurs more rarely in solid tumors, including HCC, than in hematologic malignancies. Sorafenib is administered to patients with renal impairment at recommended doses. However, the drug may cause TLS or severe renal impairment in advanced HCC patients with preexisting mild renal impairment, necessitating reduced-dose therapy and careful follow-up after the start of therapy to facilitate early diagnosis and treatment. This is a very rare complication developed in a short period treated with sorafenib as a single-agent for an advanced HCC.