ImmunoglobulinG4 (IgG4)-related sclerosing disease can occur in various organs, rarely in cardiovascular lesions. We report a case of IgG4-related cardiac tumor which was concomitant with aortic dissection. A 72-year-old woman visited our hospital with chief complaints of difficulty swallowing and weight loss. A tumor was found in the pericardium adjacent to the left ventricle on echocardiography, and a dissecting aneurysm of the ascending aorta with a maximum of 60 mm in the short diameter was noted on contrast CT. There had been no episodes related to the onset of aortic dissection such as chest pain, and chronic dissection was also identified on diagnostic imaging. A tumor biopsy was performed via a left lateral thoracotomy for tumor tissue diagnosis. Histopathologically, the tumor showed no malignant findings and we identified infiltration of IgG4-positive plasmacytes. The IgG4 level in blood exceeded the reference level, but no findings of IgG4-related disease were observed in other organs. Coronary artery CT showed the left circumflex branch of the coronary artery to run over the tumor. It was thus judged to be difficult to surgically resect the tumor. Oral steroid administration was started to reduce the size of the tumor. However, due to an enlarging trend of the aortic aneurysm involving the ascending aorta on CT at 1 month, the patient underwent ascending aorta replacement. Although infiltration of IgG4-positive plasmacytes was found in the outer lining of the resected arterial wall, the association between IgG4 and the onset of aortic dissection was unclear.