Pure mucinous breast carcinoma is a very rare histopathological type of breast cancer. Despite its invasive nature, mucinous carcinoma is most often described as a benign lump. It can be misdiagnosed as a phyllode tumor, which has certain similarities both clinically and radiologically. Surgery is the cornerstone of therapeutic management: lumpectomy or mastectomy depending on the size of the tumor, associated with axillary lymph node dissection or with sentinel lymph node biopsy procedure for early stage tumors. The treatment is often associated with adjuvant chemotherapy and radiotherapy. We describe an uncommon case of pure mucinous breast carcinoma, which clinical and radiological characteristics, occurred as a benign phyllode tumor presenting a real diagnostic challenge.