Summary Reactivation of hepatitis B virus (HBV) by reverse seroconversion (HBV‐RS) after allogeneic haematopoietic stem cell transplantation (allo‐HSCT) can occur in patients with resolved HBV infection (rHBV, defined as negative HBV surface antigen [HBsAg] and positive HBV core antibody), and may cause fatal hepatitis. To explore the risk factors, we retrospectively identified 817 consecutive patients who underwent allo‐HSCT from 2005 to 2016 in this largest single centre cohort from National Taiwan Univerisity Hospital. Transplants using donors or recipients positive for HBsAg or HBV DNA were excluded, leaving 445 rHBV patients for analysis. The 3‐ and 5‐year cumulative incidence of HBV‐RS after allo‐HSCT was 8·7% and 10·5%, respectively, at a median 16 months after allo‐HSCT. All had concurrent HBV reactivation. HBV flares developed in 19% of HBV‐RS cases, but none experienced hepatic failure. Neither did it impact non‐relapse mortality or overall survival. Multivariate analysis revealed that patients with donor lacking hepatitis B surface antibody and extensive chronic graft‐versus‐host disease (cGVHD) have the highest risk for HBV‐RS, with 5‐year incidence of 24·2%. In conclusion, adoptive immunity transfer from the donor seems to have protective effects against HBV‐RS, which may alter future donor selection algorithms, and combined with extensive cGVHD provides a good target for risk‐adaptive HBV prophylaxis.