Myeloid sarcoma is an extramedullary tumor composed of immature myeloid cells and occurs in various extramedullary sites. We report a 48-year-old man diagnosed with myeloid sarcoma in the epididymis. He was admitted to our hospital due to a painless right intrascrotal mass. Magnetic resonance imaging showed a 30 mm tumor in the right epididymis, and we subsequently performed right high orchiectomy. The pathological diagnosis was myeloid sarcoma. Bone marrow aspiration and biopsy revealed no hematological disease, and cytogenetic analysis in the bone marrow showed normal karyotype. He was diagnosed with isolated myeloid sarcoma in the epididymis. Six months after the operation, myeloid sarcoma recurred in the para aorta and left sub-diaphragm. Bone marrow examination revealed myelodysplastic syndrome, and cytogenetic analysis showed 46, XY. We performed surgical resection of the recurrent mass, and cytogenetic analysis showed 47, XY, +21. He was diagnosed with recurrent MS with adult-onset trisomy 21. Although the effect of trisomy 21 on prognosis is unknown, the patient is currently undergoing systemic chemotherapy with maintained remission.