An 80-year-old woman experienced dyspnea. Cardiac enlargement was detected by chest radiography at a local hospital. She was admitted to our hospital, and echocardiography and CT revealed pericardial effusion and multiple tumor lesions in right atrium. F-FDG PET/CT demonstrated multiple nodular accumulations in these tumors (SUVmax, 14.5). Cytologic analysis of the pericardial fluid revealed a diffuse large B-cell lymphoma. Primary cardiac lymphoma (PCL) is rare, and there are few reports about the F-FDG PET/CT imaging features of PCLs. In high F-FDG uptake in multiple tumors in the right atrium and large pericardial effusion, a PCL should be considered.