Although sarcomas are the most frequent primary malignant tumor of the heart, they very rarely involve only the left heart valves. A 72-year-old woman was diagnosed with a severe mitral valve disease and referred for cardiac surgery. Preoperative transthoracic echocardiography revealed a cardiac mass involving the mitral and aortic valves. The aortic valve had an unusual appearance, with a cystic mass protruding to the left ventricular outflow tract. The mitral valve apparatus was also involved, with chordal rupture. Extended resection and dual valve replacement was performed. A histological examination revealed an undifferentiated sarcoma. Postoperatively, neither chemotherapy nor radiotherapy therapy was used. Although most patients may benefit from adjunctive therapy, some respond well in the short term with only surgical resection, despite the poor prognosis of this rare entity.