An unusual case of primary cardiac lymphoma presenting as restrictive cardiomyopathy with arrhythmia is reported in a 72-year-old woman who was admitted for evaluation of exertional dyspnea and palpitations. Electrocardiography (ECG) showed atrioventricular dissociation and right heart cardiac catheterization revealed a typical 'dip-and-plateau' waveform. Restrictive cardiomyopathy was suspected because computed tomography (CT) did not reveal pericardial thickening, calcifications, or an effusion. Heart failure initially improved with diuretic therapy, but subsequently worsened, and the patient experienced a syncopal episode. ECG showed atrial fibrillation, and CT revealed a large mass in the right atrium and multiple tumors in the liver, which needle biopsy confirmed as diffuse large B-cell lymphoma. Chemotherapy induced complete remission, and her heart failure markedly improved. The 'dip-and-plateau' waveform was no longer detected on repeat cardiac catheterization and the ECG showed restoration of sinus rhythm. Clinically, the diagnosis was primary cardiac lymphoma.