Primary breast angiosarcoma is an extremely rare disease with a poor prognosis. Primary angiosarcoma is distinct from secondary angiosarcoma, which usually occurs in patients who have been previously treated for breast cancer. The low incidence of primary breast angiosarcoma has hindered the elucidation of its etiology and potential therapies. Here, we report a case of a patient with primary breast angiosarcoma who experienced recurrence after surgery. The tumor was refractory to systemic treatments, and the patient died 18 months after the surgery. We used RNA sequencing for gene expression profiling of the tumor. A high tumor inflammation signature score indicated enrichment in immune-related signaling. CIBERSORTx, a tool used to characterize the cellular composition of complex tissues based on gene expression, indicated that the immune cells in the tumor were predominantly macrophages, and this was confirmed using immunohistochemical analysis. These findings indicate the possible use of checkpoint immunotherapy for the treatment of primary breast angiosarcoma.