Defined as a rare extramedullary tumor, myeloid sarcoma (MS) is in the attention of specialists, although the information in the literature is represented especially through case reports. MS can precede acute myeloid leukemia (AML), appear simultaneous and can be the only manifestation of leukemia relapse after allogeneic stem cell transplantation (allo-SCT). We present the case of a 30-year-old female diagnosed with acute myelomonocytic leukemia (AML M4), with complete remission (CR) after chemotherapy, followed by allo-SCT for consolidation. After five months, the patient presented right breast tumors. Ultrasound-guided biopsy of the breast lesion displayed diffuse infiltration of undifferentiated tumor cells, with blastic granulocytic features, strongly immunopositive for cluster of differentiation (CD) 45, CD99, CD34 and myeloperoxidase (MPO) and negative for all epithelial markers [MNF116, cytokeratin 7 (CK7), estrogen receptor (ER), progesterone receptor (PR), E-cadherin]. The final diagnosis was AML relapse with breast MS. After multiple leukemia relapses with breast MS, the patient died with cerebral bleeding secondary to severe thrombocytopenia.