Reactivation of hepatitis B virus (HBV) has recently been reported as a fatal complication in patients undergoing cytotoxic chemotherapy. We herein describe a case of reactivation in a 76-year-old man who had undergone pelvic exenteration for colorectal cancer (CRC). He was treated with a modified FOLFOX6 chemotherapy regimen after the operation. Thirteen months later, his laboratory data showed severe liver dysfunction. His hepatitis B surface antigen (HBsAg) test was positive, and his HBV-DNA level was elevated. We diagnosed the patient with HBV reactivation as his HBsAg test was negative before starting chemotherapy. His liver dysfunction improved after administration of entecavir. This is the first report describing HBV reactivation following chemotherapy for an HBsAg-negative CRC patient.