Accessory breast is synonymous with polymastia or supernumerary breast tissue. An accessory breast without a nipple or areola is rare. We report a case of fibroadenoma of an accessory breast with no nipple or areola in a 41-year-old woman who presented with a right axillary mass associated with five small nodules in the normally situated breast. Magnetic resonance imaging (MRI) showed the accessory breast surrounding the tumor. We ignored the presence of the component surrounding the mass and made a preoperative diagnosis of an axillary mass of possible metastases from multiple breast cancers or breast cancer of unknown origin associated with multiple breast fibroadenomas. From a retrospective view, based on the histological results, MRI and dynamic MRI demonstrated a tiny component of breast-like tissue surrounding the axillary mass and an enhancement pattern typical of fibroadenoma for the axillary mass. For the later diagnosis of the axillary mass, the interpretation of whether the component of breast tissue surrounding the axillary mass was present is crucial. If the component exists, a tumor that originated from the accessory breast should be foremost in the differential diagnosis. Dynamic MRI appears to contribute to the diagnosis of fibroadenoma of an accessory breast before biopsy or surgical resection.