A metaplastic carcinoma of the breast (MpC) is a very rare malignant tumor accounting for fewer than 1% of all mammary tumors. MpCs are difficult to diagnose accurately and classify because of their rarity and varied histological patterns. A MpC is a histologically diverse type of malignancy in which an adenocarcinoma is found to co-exist with an admixture of spindle cells, squamous, chondroid, or bone-forming neoplastic cells. Cystic changes can be encountered in MpC, particularly in carcinoma with squamous metaplasia, but are rare in a sarcomatoid metaplastic carcinoma. We recently experienced an unusual case of a sarcomatoid metaplastic carcinoma mimicking a malignant fibrous histiocytoma of the soft tissue in a 34-year-old female, who had a breast tumor with an extensive cystic change, which was growing rapidly. Here, we report this unusual case of MpC with a review of the relevant literature.