Myeloid sarcoma is a neoplasm of myeloid cells that can arise before or concurrent with or may follow acute myeloid leukemia. Very rarely, it can present as an isolated breast lump. We have diagnosed a case of myeloid sarcoma by fine-needle aspiration cytology (FNAC), in a 52-year-old woman who presented with the right-sided breast lump. FNAC showed hypercellular smears with immature myeloid cells few neutrophils and many large round cells with high nuclear-cytoplasmic ratio, prominent nucleoli and a moderate amount of granular cytoplasm. The blast cells were myeloperoxidase positive and complete blood count, and peripheral blood examination were normal. We report this case for its rarity and as a note of caution to a pathologist to consider myeloid sarcoma in the differential diagnosis of breast lump to provide the correct diagnosis and avoid incorrect treatment of a curable disease.