Primary breast leiomyosarcoma is extremely rare. The associated imaging findings are generally not pathognomonic and can mimic those of other breast sarcomas and phyllodes tumors. The prognosis of leiomyosarcoma is better than that of other breast sarcomas. Here, we report on the case of a 36-year-old woman with a large, rapidly growing mass in the left breast, which was diagnosed as leiomyosarcoma. The lesion appeared as a circumscribed hyperdense mass on mammography, and a lobulated circumscribed mixed echoic mass with surrounding edema on ultrasonography, and it showed peripheral and multi-septated enhancement on computed tomography. We discuss the imaging findings and clinical outcome.