Sorafenib is indicated for the treatment of advanced hepatocellular carcinoma (HCC), but although rare, tumor lysis syndrome (TLS) can be fatal in HCC patients with a large tumor burden. The authors describe the case of a 55-year-old hepatitis B carrier who visited our clinic with progressive dyspnea for 3 weeks. Chest and abdominal computed tomography revealed a huge HCC in the left lobe of the liver with invasion of the inferior vena cava, right atrium, and pulmonary arteries. After 8 days of sorafenib administration, TLS was diagnosed based on the characteristic findings of hyperuricemia, hyperkalemia, and acute kidney injury with massive tumor necrosis by follow-up imaging. Despite discontinuation of sorafenib and supportive care, the patient's clinical course rapidly deteriorated. The authors describe a rare but fatal complication that occurred soon after sorafenib initiation for HCC. Careful follow-up is required after commencing sorafenib therapy for the early diagnosis and management of TLS.