Background: Angiosarcoma of the breast is a rare form of breast malignancy. Primary breast angiosarcoma occurs sporadically primarily in young women while secondary breast angiosarcoma most frequently develops in older patients with prior history of radiation therapy or chronic lymphedema. This aggressive malignant tumor of the vascular endothelium has a high risk of local recurrence and distant metastasis and the diagnosis carries a poor prognosis.Case Description: We present a case of a 46-year-old female with a palpable left breast mass that initially masqueraded as a benign vascular lesion on core needle biopsy. After the mass began to enlarge rapidly, an excisional biopsy was performed revealing primary breast angiosarcoma and the patient was definitively managed with mastectomy. Additional surgical excision was required to achieve negative margins.Conclusions: Diagnosis of primary angiosarcoma of the breast is made difficult by an insidious clinical onset, no pathognomonic radiographic features and deceptively benign histologic appearance on biopsy. Surgical resection is often necessary to obtain sufficient tumor sampling for microscopic examination to render a final diagnosis. In young patients, a rapidly growing, highly vascular breast mass should be considered angiosarcoma until proven otherwise. Given the lack of consensus regarding systemic therapies, complete surgical resection of the tumor with mastectomy is the primary curative treatment option. A multimodal treatment approach should be considered in high-risk patients with large, advanced tumors in collaboration with a multidisciplinary care team.