<b><i>Background:</i></b> Malignant phyllodes tumors of the breast are rare biphasic neoplasms. Only few cases related to pregnancy have been reported. <b><i>Case Report:</i></b> A 37-year-old woman presented with swelling and pain in her left breast as well as hyperemia on the breast skin, 4 weeks after labor. In her family history, her aunt and maternal cousin had had a breast cancer diagnosis. Clinical evaluation of the patient was consistent with a breast abscess. Therefore, abscess drainage and biopsy from the cavity wall were performed. However, the biopsy was diagnosed as malignant phyllodes tumor. An evaluation by ultrasonography showed a well-defined hypoechoic mass with many cystic spaces covering the entire breast tissue. Therefore, a simple mastectomy was performed. Microscopic examination revealed a high-grade malignant phyllodes tumor. Additionally, bone cyst-like areas in the form of sponge-like blood-filled non-endothelialized spaces were observed. <b><i>Conclusions:</i></b> Since the breasts become larger due to the physiological changes during pregnancy, any underlying breast lesions may be obscured. Therefore, clinical breast examination in the first visit of pregnancy is important.