Mastocytosis is a rare condition in which mast cells accumulate throughout various organs of the body-the most common subtype being confined to the skin. We present an unusual case of cutaneous mastocytosis localized to the unilateral breast of a young woman with partial involvement of the areola. Previously diagnosed as nipple eczema, the patient failed appropriate treatment with class III and IV topical corticosteroids. Given it was adult onset, failed appropriate treatment, and had an atypical clinical appearance, a biopsy was pursued that revealed mastocytosis in skin. This is another clinical diagnosis dermatologists may consider in their differential diagnosis of nipple dermatitis.