Allogeneic hematopoietic stem cell transplantation (HSCT) is a curative therapy for various kinds of hematological malignancies and disorders. Recently, HLA-haploidentical stem cell transplantation with post-transplantation cyclophosphamide (PTCy-haplo HSCT) has been widely performed due to its safety and favorable immune recovery. However, graft rejection remains an obstacle to PTCy-haplo HSCT. Donor specific antibody (DSA) is considered to be a major factor of graft rejection of haplo HSCT. We herein present a case of graft rejection after PTCy haplo-HSCT due to DSA induced by pretransplant platelet transfusion after donor selection. The patient was dependent on platelet transfusion and had not received cytotoxic chemotherapy because he was diagnosed as myelodysplastic syndrome/myeloproliferative neoplasm-unclassifiable. We retrospectively confirmed the level of DSA just before the first transplantation and found that it was dramatically elevated, which was enough to cause graft rejection. We successfully performed cord blood transplantation of the HLA that was not the target of DSA, as salvage transplantation without any desensitization. This case illustrates that we have to confirm the presence of DSA immediately before the haplo-HSCT, particularly in high risk patients who are dependent on platelet transfusion and have no cytotoxic chemotherapy before HSCT.