Background: Primary actinomycosis of the breast is a rare disease which may present as sinus tract or with mass-like features mimicking malignancy. Clinical presentation makes it difficult to distinguish primary actinomycosis from mastitis and inflammatory carcinoma. Case Report: A 48-year-old woman presented with a mass in the left breast of 2 months duration. Physical examination was significant for a non-tender mass in the left breast. Histopathologic examination of the excisional biopsy of the mass showed granulomatous inflammation with grains of Actinomyces israelii. Conclusions: Actinomycosis of the breast usually presents as a recurrent abscess with fistulas. It may sometimes present as a breast lump, which is difficult to distinguish from inflammatory carcinoma. The diagnosis is made by histopathologic examination of the specimen, in which we can see the characteristic sulfur granules representing the bacterial colonies. Prolonged antibiotic therapy with penicillin is the treatment of choice.