Primary giant cell tumor of soft tissue (GCT-ST) arising in breast is exceedingly rare. We report a case of a 60-year-old woman with a primary breast giant cell tumor that appeared histologically identical to giant cell tumor of bone and had a clinically malignant course. The patient presented with a cystic mass of the breast, suspected on imaging to be an organizing hematoma, possibly related to previous injury. Histopathological evaluation revealed a neoplasm composed of mononuclear cells admixed with osteoclast-like giant cells resembling giant cell tumor of bone. Immunohistochemical staining was positive for CD68, smooth muscle actin, and vimentin, but was negative for a panel of epithelial and additional muscle markers. These features were most consistent with GCT-ST, an uncommon neoplasm of low malignant potential. Despite aggressive surgical treatment achieving clear surgical margins, the patient expired with pulmonary metastases within a year of her initial presentation. This case demonstrates the difficulty of predicting clinical behavior of GCT-ST of breast on the basis of histological features and depth of tumor alone. To our knowledge, this is the first case report of a GCT-ST arising in the breast associated with a fatal outcome. The distinction of this entity from other more common primary breast tumors with giant cell morphology is also emphasized.