Humoral hypercalcemia of malignancy (HHM) is a paraneoplastic syndrome primarily caused by a tumor-producing parathyroid hormone-related protein (PTH-rP). We describe the first reported case of a uterine carcinosarcoma causing HHM. A 70-year-old patient was transferred to our hospital for a uterine tumor accompanied by impaired consciousness. The laboratory tests indicated anemia, malnutrition, elevated serum calcium and elevated PTH-rP. Emergency surgery, including abdominal hysterectomy and bilateral salpingo-oophorectomy, was performed due to uncontrollable uterine bleeding. The pathological diagnosis was carcinosarcoma consisting of pure squamous cell carcinoma in its epithelial component. Postoperatively, chemotherapy with paclitaxel and carboplatin was performed. The patient had recurrent tumors at the para-aortic lymph nodes 11 months after the initial surgery and underwent a pelvic and para-aortic lymphadenectomy, which removed all of the recurrent tumors.