Primary cardiac lymphoma is an uncommon malignancy, accounting for 1.3% of primary cardiac tumors and 0.5% of extranodal lymphomas. But, secondary involvement of the heart is seen in 8.7-27.2% of the documented clinical cases of lymphoma. A 66-year-old man was referred to our ER for evaluation of dyspnea. We incidentally detected the huge intracavitary mass of the right atrium using transthoracic and transesophageal echocardiography. Emergently, the mass was surgically resected due to the possibility of sudden death because the mass was at the risk of obstruction of the right ventricular outflow tract. The mass was confirmed with malignant lymphoma (diffuse large B cell type). We report a case of intracavitary cardiac involved huge non-Hodgkin's lymphoma detected using transthoracic echocardiography in a patient complained of dyspnea.