Malignant cardiac tumors are extremely rare, representing only 25% of all cardiac tumors, and angiosarcoma is the most common subtype. Myxofibrosarcomas are one of the rarest forms of cardiac malignant tumors. These tumors can silently grow and produce no or few symptoms until the tumor is large enough to obstruct blood flow. The definitive treatment is negative margin resection, if possible. Most cardiac tumors have a limited response to chemotherapy and radiotherapy. Therefore, surgical treatment is considered the best option. Our patient is a 57-year-old Caucasian postmenopausal female who presented with dyspnea, shortness of breath, and fatigue. Echocardiography confirmed the presence of a mass in the left atrium. A median sternotomy was performed with aortic and bicaval cannulation. Right atriotomy was performed, and the left atrium was exposed through the transseptal approach. A rounded smooth-surfaced mass was found in the left atrium that was 3.5 × 4.5 cm in size. The mass had a prominent and firm attachment point with a wide-based stalk in the pulmonary vein-right atrium border area. The tumor was completely excised, and the excision point was strengthened with a running suture. The following pathologic diagnosis was finally made: myxofibrosarcoma, FNCLCC (National Federation of Cancer Centres) Grade 2. Immunohistochemistry showed positivity for Epithelial membrane antigen (EMA), desmin, calretinin, Ki67, Smooth Muscle Actin (SMA), and S100. Given the rarity of cardiac malignant tumors, we thought preoperatively that this particular tumor could be a myxoma although it did not have the classical attachment point with a stalk at the interatrial septum. Our case is an example of how fast a cardiac sarcoma can grow. Nine months before the presentation, the patient underwent an echocardiography that did not show any signs of tumor growth. The estimated time of growth was 9 months or less. We followed our patient, performing a computer tomography scan and echocardiography 1 month after surgery, and these did not show any signs of tumor growth.