One significant complication of hepatitis B virus includes reactivation (HBVr) in the context of the use of immunosuppressive agents, such as corticosteroids and rituximab, among others. Limited data exist on the topic of HBVr risk in the context of tyrosine kinase inhibitors for which there is no strong guidance recommendation. We describe the clinical characteristics, diagnostic challenges, and the clinical course of a single patient with recurrent mantle cell lymphoma who developed HBVr after treatment with acalabrutinib, a Bruton tyrosine kinase inhibitor.