Sarcomas of the breast are rare and comprise less than 1% of mammary cancers. Myxofibrosarcoma as an unusual variant of malignant fibrous histiocytoma in the breast and during pregnancy has not previous been reported in the literature. We present the case of a 32-year-old woman with a rapidly growing subcutaneous left breast mass that appeared during her first pregnancy. After ultrasonography and core needle biopsy, she underwent a modified radical mastectomy. Pathologic examination revealed a multinodular subcutaneous mass with characteristic microscopic features consistent with myxofibrosarcoma. Immunohistochemistry findings supported the diagnosis and excluded other differential diagnoses. There has been no consensus about proper management and surveillance in these rare groups of malignancies, however wide local excision and adjuvant treatment for moderate and high grade tumors are recommended.