Lupus mastitis is an uncommon presentation of lupus erythematosus profundus or lupus panniculitis, a rare variant of lupus erythematosus characterized by inflammation of the subcutaneous fat. Lupus mastitis can present as single or multiple subcutaneous or deep breast masses, often clinically mimicking malignancy. Although lupus mastitis is rare, with less than 25 cases reported, the histologic features are distinct. Awareness of the entity and familiarity with the histologic features allow for accurate diagnosis and appropriate patient management. It most commonly affects women with a mean age at diagnosis of 40 years and an age range of 18 to 70 years. Typical histologic findings in lupus mastitis include a lymphocytic lobular panniculitis with plasma cells and hyaline fat necrosis. The lymphocytic infiltrate can be nodular, diffuse, periductal, and/or perilobular and germinal centers can frequently be identified. Lymphocytic vasculitis is also common. Immunohistochemistry shows a mixed T and B-cell population, with predominantly CD3+CD4+ T cells intermixed with CD20-positive B cells and polyclonal plasma cells. Most commonly, lupus mastitis is seen in patients with a previous diagnosis of systemic or discoid lupus; however, it can also be the initial presentation of lupus in some patients.