Primary mucinous cystadenocarcinoma (MCA) of the breast is a rare variant of breast carcinoma known to have a favorable prognosis despite showing a basal-like phenotype. We describe a case of MCA breast in a 45-year-old female with a palpable mass in the breast. On the basis of the histopathological and immunohistochemical evaluation of a lumpectomy specimen with the absence of mass anywhere else on whole-body imaging, a diagnosis of primary MCA was rendered. Mismatch repair protein evaluation showed this tumor to be microsatellite stable. Molecular testing revealed the absence of Kirsten rat sarcoma viral oncogene homolog (KRAS), neuroblastoma RAS viral oncogene homolog (NRAS), and v-raf murine sarcoma viral oncogene homolog B1 (BRAF) mutations. To date only 27 cases of MCA breast have been reported. To the best of our knowledge, ours is the first case documenting diffuse Cytokeratin 20 (CK20) positivity, microsatellite stability, and the absence of KRAS, NRAS, and BRAF mutations in these tumors. The rarity of this tumor further evokes an interest in this case. A better understanding of the disease warrants a review of more cases with longer follow-ups.