Primary cardiac lymphoma (PCL) has rarely been reported in Chinese populations. PCL mostly occurs in the right atrium. The clinical manifestations may be variable and are attributed to its location, the presence of congestive heart failure, pericardial effusion, arrhythmia, and cardiomegaly. The prognosis is usually poor because it is usually found too late and therefore, clinicians should be aware of PCL. Imaging examinations are the best methods for initial diagnosis and include echocardiography, computed tomography (CT) scan, magnetic resonance imaging (MRI), and radioisotope scan. However, the final diagnosis is made by pathology, such as cytologic examination of the effusive fluid and tissue biopsy. Because the tumors are difficult to resect, the main treatment for the disease is chemotherapy, which can be successful. Here, we report a 58-year-old man who had a tumor measuring 8 × 5 cm in the right atrium. By clinical staging, including chest X-ray, echocardiography, CT scan of the abdomen, MRI of the heart, whole body tumor Gallium scan, and gastrointestinal series, no metastatic lesion or involvement was found in other parts of the body. Pathologic findings including cytology of pericardial effusion and heart tumor biopsy revealed the case as a diffuse large B-cell lymphoma. After chemotherapy with COP (cyclophosphamide + vincristine + prednisone) and CHOPBE (COP + doxorubicin + bleomycin + etoposide) regimens, the intracardiac tumor had disappeared, but the patient survived for 12 months in total, despite additional radiotherapy over the pericardial lesions. It was presumed that because the tumor was very large and involved all 3 layers of the heart, it did not respond as well to the therapy as expected.