Primary cardiac tumors, which are uncommon types of tumors, can be presented with a variety of clinical signs and symptoms, depending on their location. We present a case of a 57-year-old female patient with a severe right-sided heart failure. Examination using 2-dimensional transthoracic and 3-dimensional transoesophageal echocardiography detected a large, oval, tumor-like formation within the right atrium, which compromised the blood flow from the superior and inferior vena cava. It appeared to have an irregular echo-free space in its central part, probably due to necrosis. Thoracic multislice computed tomography revealed a heterogeneous, expansive, tumor-like mass in the right atrium, with signs of bleeding in its center. Although there were no signs of metastatic dissemination, it could not be excluded that the tumor-like mass originated outside of the heart. The patient underwent surgical resection of the tumor. The surgery was accompanied with bleeding complications that developed due to the central necrosis with local infiltration. During the postoperative period, severe systemic inflammatory response syndrome developed and the patient died. Pathologists diagnosed undifferentiated pleomorphic cardiac sarcoma for which the prognosis is usually poor. The median survival of patients with this type of diagnosis is less than 1 year, even with surgical resection and further adjuvant therapy.