Primary leiomyosarcoma of the breast is an extremely rare malignancy, accounting for less than 1% of all breast tumors. Diagnosis is challenging because its morphology overlaps with that of other spindle cell lesions, and standardized treatment guidelines are currently unavailable. A 44-year-old woman presented with a rapidly enlarging, firm, 15×15 cm mass in the left breast. She had previously undergone surgery elsewhere, with diagnoses of leiomyoma and desmoid-type fibromatosis. Imaging demonstrated a lobulated mass without evidence of metastasis. Excision revealed pleomorphic spindle cells arranged in intersecting fascicles, with necrosis and dermal invasion. Primary breast leiomyosarcoma was confirmed by immunohistochemistry, which demonstrated smooth muscle actin and desmin positivity and negative staining for pancytokeratin, p63, S100, CD34, and BCL2. The patient underwent modified radical mastectomy followed by adjuvant radiotherapy and chemotherapy. Primary breast leiomyosarcoma is a rare entity that remains diagnostically challenging. Immunohistochemistry is essential for accurate diagnosis, and optimal management requires a dedicated multidisciplinary approach.