We present a very rare case of nodular mucinosis of the breast. A 30-year-old woman noticed a right breast lump and consulted at our hospital because it gradually increased in size. On physical examination, the lump was 30 x 25 mm in size, and was located in the upper outer quadrant close to the nipple of the right breast. It was well-demarcated, mobile and hard. Ultrasonography (US) showed a clearly circumscribed, lobulated, and homogeneous hypoechoic lesion. Mammography (MMG) showed a round-lobular-shaped radiopaque mass without microcalcifications or spicula formation. Fine needle aspiration cytology (FNA) revealed no malignancy and mucin. Histologically, the excised tumor consisted of an abundant myxoid substance with scattered spindle cells without epithelial elements in the mucous lake. The mucinous substance stained positively with Alcian blue. Nodular mucinosis, simulating mucinous carcinoma or phyllodes tumor on clinical and imaging examinations, should be included in the differential diagnosis in cases of mucinous lesions occurring near the nipple in a young woman.