Malignant mesothelioma of the pleura often involves the heart but seldom invades the intracardiac cavity. We report a 78-year-old woman with right pleural mesothelioma who died of heart failure. An autopsy revealed that the tumor was present at the right pleura and invaded the right upper lobe of the lung and the mediastinum. The tumor also extended to the left atrium via the right pulmonary vein and filled the atrial cavity. Repeated transthoracic echocardiography failed to detect the tumor, but magnetic resonance imaging was useful for diagnosis.