The authors describe a case of undifferentiated pleomorphic sarcoma of the breast occurring in a 50-year-old woman who presented with a palpable mass in her right breast. She first noticed the mass one month previously. Core needle biopsy showed connective tissue including epithelioid and spindle cells. The patient underwent total mastectomy without axillary lymph node dissection. Based on examination of the excised tumor, the initial pathologic diagnosis was atypical spindle-shaped and ovoid cells with uncertain malignant potential. Histological findings with immunomarkers led to the final diagnosis of undifferentiated pleomorphic sarcoma.This case highlights a rare and interesting variant of primary breast sarcoma and the important role of immunohistochemistry in defining histological type and differential diagnosis. Hence, undifferentiated pleomorphic sarcoma has been a diagnosis of exclusion performed through sampling and critical use of ancillary diagnostic techniques.