Endometrial stromal sarcoma (ESS) is a rare neoplasm of uterine origin. Intracardiac metastasis from this tumor is extremely infrequent. This report describes a 24-year-old woman from Yemen who had irregular vaginal bleeding shortly after spontaneous abortion. She developed left-lower-limb swelling, diagnosed by duplex scanning and magnetic resonance imaging as deep venous thrombosis in the inferior vena cava (IVC) that extended into the iliac veins on both sides, as well as the left femoral vein and right atrium. She developed acute respiratory distress, from which she recovered after transfer to the intensive care unit. Transesophageal echocardiography showed a large mass occupying the right atrium and ventricle and another mass in the right ventricular outflow tract with areas of cavitations. The tumor appeared to come from the IVC and extended through the right atrium and right ventricle into the pulmonary artery, ending in several digit-like projections. After surgical resection of the intracardiac mass, pathologic examination revealed a low-grade ESS that was confirmed by immunohistochemistry. The patient underwent panhysterectomy and IVC debridement. Pathologic examination revealed infiltrating low-grade endometrial sarcoma invading the myometrium and left adnexa, with intravenous extension into the pelvic veins and the IVC to the right side of the heart. This case shows that despite its well-known good prognostic nature, low-grade ESS may behave as an aggressive malignancy.