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Abstract Background IgG4-related diseases are characterized by marked infiltration and IgG4-positive plasma cells and fibrosis, and involve multiple organs. However, IgG4-related mastitis is rare. We report a case of mastitis associated with IgG4-related disease. Case presentation A 78-year-old woman visited our hospital with a complaint of multiple subcutaneous nodules. A biopsy of a dorsal subcutaneous mass was performed but did not yield a definitive diagnosis. However, blood tests showed a high level of IgG4, thus she was referred to the department of collagen disease for further examination. Computed tomography (CT) showed a thickening of the skin of the right breast, and the patient was referred to our department. On physical examination, a large area of thickened skin was observed in the right breast without inflammatory breast cancer-like redness, and no mass was palpable. A needle biopsy was performed on an indistinct hypoechoic area in the breast, and she was diagnosed with mastitis associated with IgG4-related disease. Systemic steroid therapy was then administered and the symptoms of multiple skin nodules and mastitis improved. Conclusions We reached the diagnosis based on a biopsy of the mammary gland enabling the patient to begin treatment for IgG4-related disease. This case was characterized by breast skin thickening, which is different from inflammatory breast cancer.