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Key Clinical Message: A unique ER/PR‐positive mucinous adenocarcinoma of breast but CK7/CK20 negativity. A rare finding of Lymphoplasmacytic Infiltration is noted. Successful treatment included modified radical mastectomy and tamoxifen‐based therapy, leads to remission. Mucinous carcinoma, which is characterized by the presence of abundant extracellular mucin, is a rare and distinctive subtype of invasive breast cancer. This subtype accounts for less than 5% of all invasive breast cancers, with pure mucinous carcinoma representing only around 2%. It is most commonly found in older patients, typically in the sixth to early eighth decade of life. We present the case of a 55‐year‐old female with a 15‐year history of a painless, gradually enlarging palpable lump on her left breast. Upon examination, a 5 × 5 cm lump with well‐defined margins and a hard consistency was found in the lower outer quadrant of the left breast. Histopathological examination confirmed a diagnosis of Mucinous Adenocarcinoma of the Left Breast. The patient underwent modified radical mastectomy and received adjuvant endocrine therapy with tamoxifen due to hormone receptor‐positive status. Mucinous carcinoma of the breast is a unique entity with specific histological features including the presence of extracellular mucin pools. Immunohistochemical staining is crucial for determining hormone receptor status, which can guide treatment decisions. Although surgical intervention is the primary approach, the extent of surgery may vary, ranging from lumpectomy to mastectomy. Adjuvant therapies such as chemotherapy and radiotherapy may also be considered based on individual cases. This case underscores the importance of a multidisciplinary approach in managing rare subtypes of breast cancer, such as mucinous carcinoma. Accurate diagnosis, appropriate surgical intervention, adjuvant therapy, and long‐term follow‐up are critical components of treatment. [ABSTRACT FROM AUTHOR]