ABSTRACT Matrix-producing carcinoma (MPC) of the breast is relatively rare. We report on a case of MPC of the breast during lactation. A female in her 30s noticed a lump in the upper inner and outer quadrants of her left breast 9 months into her pregnancy. She came hospital at one month after delivery. Ultrasonography revealed a hypoechoic tumor with an unclear border region. An MRI revealed a breast tumor approximately 19 mm in size, with dynamic studies demonstrating early contrast and ring enhancement. A core needle biopsy was performed, resulting in an histopathological diagnosis of invasive carcinoma Estrogen receptor-, Progesterone receptor-, Human epidermal growth factor receptor 2(HER2)-; stage I, T1cN0M0. Breast-conserving surgery and a sentinel lymph node biopsy (SLNB) were performed. The SLNB was negative for cancer. Chemotherapy (FEC100, DTX75) and radiotherapy were performed as adjuvant therapies. Seventeen months after surgery, the patient is recurrence-free.