Hepatitis B virus (HBV) reactivation can occur in immunosuppressed patients. Specifically, HBV reactivation after chimeric antigen receptor T-cell (CAR T-cell) therapy is a known complication with few case reports and specific treatment guidelines. Our patient experienced HBV reactivation 27 months after CAR T-cell therapy even with 23 months of entecavir prophylaxis. This unique case highlights the need for further investigation into the risk of HBV reactivation after CAR T-cell therapy and the proper HBV prophylaxis during and after CAR T-cell therapy.