Primary cardiac osteosarcomas are uncommon tumors. Their growth pattern is aggressive and the prognosis is poor. A 34-year-old, 7-month pregnant woman with a primary cardiac osteosarcoma associated with the anterior mitral leaflet and connected to the interatrial septum, causing nearly subtotal obstruction of the mitral valve, presented with a clinical picture of heart failure. Transthoracic echocardiography demonstrated an abnormal mass located in the mitral anterior leaflet, prolapsing into the left ventricle. Radical resection of the mass and replacement of the mitral valve were performed under cardiopulmonary bypass. Histopathology showed a low-grade osteosarcoma. The postoperative course was uneventful, and the patient was discharged on the 16th day after surgery. She gave birth after 2 months to a healthy baby after the successful operation. She and her baby did not have any problem during 3 months of follow-up.