B-cell prolymphocytic leukemia (B-PLL) is a rare, chemotherapy-resistant lymphoid neoplasm. Although allogeneic hematopoietic stem cell transplantation (allo-HSCT) represents a potentially curative treatment for B-PLL, the number of B-PLL patients who have been treated with allo-HSCT is small and its efficacy has not been established. We report the case of a 59-year-old woman with B-PLL in partial remission, who was successfully treated with allo-HSCT following reduced-intensity conditioning (RIC). She was initially treated with four courses of combination chemotherapy consisting of rituximab, fludarabine, cyclophosphamide, and mitoxantrone, which resulted in partial remission with residual tumor cells comprising 7 % of bone marrow nucleated cells. Although the residual B-PLL cells appeared to be indolent, as the disease progressed slowly during partial remission, these cells lost CD20 expression, and the prognosis of the patient was considered to be poor with conventional chemotherapy. She was therefore given RIC, followed by allo-HSCT from an HLA-matched sibling donor. Her clinical course following allo-HSCT was uneventful, and she remained in complete remission at 32 months post-transplantation. Although the therapeutic strategy for B-PLL should be determined based on the severity of the disease, RIC with allo-HSCT may be a therapeutic option for indolent B-PLL when the long-term prognosis of patients is markedly poor.