Background Hepatitis B virus (HBV) reactivation is commonly seen in HBsAg-positive hematologic patients undergoing immunosuppressive chemotherapy. Little is known about the risk of HBV reactivation after chimeric antigen receptor T-cell (CAR T) immunotherapy for the treatment of refractory/relapsed malignant B-cell lymphoma. Case presentation We report a patient who underwent antiviral prophylaxis for 26 months and who discontinued treatment by herself 1 month after the sequential infusion of two specific, third-generation anti-CD19 and anti-CD22 CAR T cell immunotherapies for refractory/relapsed diffuse large B-cell lymphoma. Remission of the primary disease was achieved after two and half months, but she was admitted with a 7-day history of vomiting, jaundice, itching and dark urine. After excluding other possible causes of acute liver damage, HBV reactivation was suspected. HBV-DNA was 4,497,000 IU/mL at that time. Following the reintroduction of entecavir, a decline in the HBV-DNA copies was observed, but ALT, AST and bilirubin were elevated, and there was no improvement of the clinical conditions. She passed away because of hepatic encephalopathy and multiple organ dysfunction syndrome 40 days after admission. Conclusions Our study provides the first report of the severe, early reactivation of an inactive HBsAg carrier after CAR T cell therapy in DLBCL. Trial registration: ChiCTR-OPN-16008526.