Primary malignant mesenchymoma involving the heart is extremely rare. It usually has a poor prognosis. We report a case of malignant mesenchymoma originating in the left atrium with protrusion into the right pulmonary veins. Clinically, this patient presented with congestive heart failure, hemoptysis and systemic arterial embolization. Transthoracic and transesophageal echocardiography revealed a huge mass originating in the posterior wall of the left atrium with extension into both upper and lower right pulmonary veins. An elevated pulmonary arterial wedge pressure, moderate pulmonary hypertension and stenosis of the lower abdominal aorta were found during cardiac catheterization. Magnetic resonance imaging with angiography disclosed a tumor mass over the abdominal aorta above the bifurcation with protrusion into the right common iliac artery. This patient underwent surgical resection of the cardiac tumor and postoperative adjuvant chemotherapy. The pathological finding of a tumor containing several cellular types of sarcoma confirmed the diagnosis of malignant mesenchymoma.