This is a case report of a six-year-old female White Alpine sheep with a cardiac fibrosarcoma in the right atrium. Clinically, the sheep had right-sided cardiac insufficiency with tachycardia, engorgement of the jugular veins, brisket edema, and ascites. Chronic congestion of the liver resulted in increased hepatic enzyme activity. Based on clinical findings, a tentative diagnosis of endocarditis or pericarditis was made. Radiography of the thorax revealed hydrothorax. An echogenic mass was observed in the right atrium via echocardiography; it was interpreted as a tumor or thrombus. Ultrasonography of the abdomen revealed severe ascites and chronic congestion of the liver attributable to right-sided cardiac insufficiency. The clinical and sonographic findings were verified at post mortem. The mass in the right atrium was a pedunculated fibrosarcoma.