Right atrial cardiac osteosarcoma is a rare intracardiac tumor that can be difficult to diagnose. This report describes a 20-year-old man who was referred to our hospital with symptoms of heart failure. Echocardiography revealed a mass adhering to the right arterial septum that extended into the right ventricle and inferior vena cava. Right ventricular failure and worsening symptoms indicated the need for emergency life-saving surgery. Thereafter, the associated symptoms dramatically and rapidly diminished, and two days later the patient had recovered sufficiently to walk unaided. The short-term results of surgical tumor excision were excellent.