The first case of plasma cell granuloma (PCG) of the breast is presented. The lesion was seen as a circumscribed firm mass in the right breast of a 29-year-old woman. Histologically, a prominent plasma cell infiltrate was seen in a fibrous hyalinized stroma that contained spindle- and large stellate-shaped stromal cells with bizarre nuclei. Immunohistochemical study demonstrated the polyclonal nature of the plasma cells and characterized the stromal cells as myofibroblasts. The nature of these cells was confirmed ultrastructurally. PCG of the breast should be considered analogous to inflammatory pseudotumors of the lung and extrapulmonary sites.