Undifferentiated uterine sarcoma (UUS) is a high-grade sarcoma with no specific differentiation; however, it exhibits variable positivity for CD10 as an immunohistochemical marker of endometrial stroma, suggesting immunohistochemical differentiation into endometrial stroma. It has been reported that some low-grade endometrial stromal sarcoma pulmonary metastatic foci show cystic changes; however, whether pulmonary metastatic UUS foci are solid or cystic remains undescribed. A 63-year-old woman underwent a hysterectomy. The subsequent pathologic diagnosis was UUS. After the operation, she was treated with chemotherapy; however, pulmonary solitary shadow was radiologically detected. Eight months after the gynecologic operation, her pulmonary lesion was surgically resected. Pathologically, the lesion consisted of a cyst containing serous fluid with thick whitish wall composed of tumor cells, which led to the diagnosis of metastatsis of UUS. The cyst seemed to be formed via secondary degeneration. She remains healthy without recurrence 28 months after her first visit to our hospital.