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Ibrutinib has been a first-line treatment for chronic lymphocytic leukemia since 2014. Case reports of hepatitis B virus (HBV) reactivation after ibrutinib initiation have been presented. The association between the risk of HBV reactivation and ibrutinib initiation remains unclear. This nationwide study aimed to estimate the incidence of HBV reactivation after ibrutinib initiation.This study included patients who received ibrutinib between 1 February 2014 and 31 October 2019. Possible reactivations were searched by (1) changes in HBV surface antigen or HBV DNA from no data or negative status to positive after ibrutinib initiation, (2) alanine aminotransferase levels that were at least 3 times the baseline value after ibrutinib initiation, and (3) new antiviral prescriptions against HBV after ibrutinib initiation. Individual chart reviews were conducted to identify HBV reactivation attributed to ibrutinib. The cumulative incidence of HBV reactivation was calculated.A total 4130 patients were eligible during the study period. Of these, patients with negative HBV core antibody (anti-HBcAb; n = 1670) and patients who were taking antivirals against HBV (n = 60) were excluded. There were 2219 patients without anti-HBcAb testing results. Among the remaining 181 patients with positive anti-HBcAb, 7 HBV reactivations were directly attributable to ibrutinib treatment after chart review, for a 3.9% cumulative incidence.Our study revealed a low cumulative incidence of HBV reactivation after ibrutinib initiation among patients with previous anti-HBcAb positivity, indicating a moderate risk of HBV reactivation.