The presence of supernumerary breast (or polymastia) is not uncommon; however, reports of fibroadenoma in supernumerary breast tissue are rare. The present article reports a case of fibroadenoma developing in the supernumerary breast located below the normal left pectoral breast in a 26-year-old woman. Clinical and ultrasonographic examination of both pectoral breasts revealed no abnormalities but a mass was found in the third breast. The patient underwent resection of the supernumerary breast and the histopathological examination showed an intracanalicular type of fibroadenoma within the breast tissue.