Hepatitis B virus (HBV) reactivation during anticancer chemotherapy or immunosuppressive therapy in chronic carriers can lead to fatal liver failure. We report a rare case of severe HBV reactivation during postoperative radiotherapy with concomitant and adjuvant temozolomide (TMZ) for malignant glioma. A 49-year-old Japanese woman with a history of HBV carrier status with positive results for hepatitis B surface antigen presented with persistent headache due to a tumor in the left frontal lobe. The tumor was partially resected and anaplastic astrocytoma was diagnosed. Postoperative liver function was normal and radiotherapy plus concomitant and adjuvant TMZ was started. Impaired liver function became apparent just before administration of adjuvant TMZ, and acute liver failure developed. Antiviral therapy including entecavir, a nucleoside analog, led to a successful outcome and the patient survived. This case underlines the possibility of HBV reactivation due to TMZ and suggests the utility of HBV screening and antiviral prophylaxis before administration of TMZ to patients with malignant glioma.