We report a case of donor-derived diffuse large B-cell lymphoma (DLBCL), which developed 5 years after stem cell transplantation from a human leukocyte antigen (HLA)-haploidentical donor for acute myeloid leukemia (AML). A 51-year-old male was diagnosed with AML with variant KMT2A translocation involving t(6;11)(q13;q23). After 12 cycles of azacitidine treatment, fluorescence in situ hybridization (FISH) for KMT2A split signal indicated that 94% of his bone marrow (BM) cells were positive. He underwent peripheral blood stem cell transplantation (PBSCT) from his HLA-haploidentical son. The preconditioning regimen consisted of fludarabine, busulfan, melphalan, and antithymocyte globulin (ATG). The graft-versus-host disease (GVHD) prophylaxis consisted of tacrolimus and short-term methotrexate. On day 28, KMT2A FISH analysis indicated that he had achieved a complete response (CR). He continued to receive tacrolimus for the limited type of cutaneous chronic GVHD. Five years after the transplantation, positron emission tomography/computed tomography (PET/CT) showed an abdominal tumor. The tumor was diagnosed as DLBCL without Epstein-Barr virus. BM aspiration revealed the infiltration of lymphoma cells with t(8;14)(q24;q32). Chimerism analysis showed that both the peripheral blood (PB) and abdominal lymphoma cells were of donor origin. After 4 cycles of salvage chemotherapy, PET/CT showed that a CR had been achieved. He underwent a second PBSCT from an HLA-identical unrelated donor. The preconditioning regimen and GVHD prophylaxis were the same as those for the first PBSCT without ATG. The patient's PB revealed complete second donor-type chimerism, and the patient has maintained a CR since the second transplantation.