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Primary breast leiomyosarcoma is a rare mesenchymal malignancy, thought to arise from smooth muscle elements of the nipple-areolar complex, vascular structures, or stromal tissue. Owing to its low incidence, current knowledge is largely derived from isolated case reports and small case series. Patients typically present with a painless, gradually enlarging breast mass, while imaging findings are often non-specific, posing diagnostic challenges. Definitive diagnosis relies on histopathological evaluation supplemented by immunohistochemistry (IHC) to differentiate it from other spindle cell neoplasms of the breast. Treatment strategies are extrapolated from the management of soft-tissue sarcomas at other sites, with wide local excision and negative surgical margins forming the mainstay of therapy. The role of adjuvant radiotherapy and chemotherapy remains unclear due to limited evidence and a lack of consensus guidelines. Given its rarity, reporting such cases is essential to enhance understanding of its clinical behavior, optimize management approaches, and inform future therapeutic recommendations. A patient in her 30s, status post a simple mastectomy two years ago (histopathological diagnosis: borderline malignant phyllodes tumor), presented with a rapidly growing right chest wall swelling. Ultrasound of the swelling showed an oval, well-circumscribed, heterogeneous lesion measuring 3.9 cm × 2.8 cm, with solid and cystic components in the right chest wall. Fine-needle aspiration showed clusters of spindle cells with nuclear atypia. Wide local excision demonstrated a high-grade spindle cell tumor, confirmed by histopathology and IHC (smooth muscle actin positive, Ki-67 >10%) as leiomyosarcoma. On follow-up, no evidence of residual disease or distant metastasis was identified. This case highlights the challenges in diagnosing rare breast sarcomas, the potential for recurrence or transformation of phyllodes tumors, and the reliance on complete surgical excision followed by adjuvant radiotherapy for optimal local disease control.