Malignant cardiac lymphoma is rare and commonly involves nodules on the right side of the heart. We herein report a case of malignant cardiac lymphoma with diffuse extension into the left ventricle. The patient was a woman in her 60s who complained of dyspnea and malaise. Echocardiography revealed left ventricular hypertrophy (LVH), and magnetic resonance imaging revealed diffuse contrast enhancement on delayed contrast. Cardiac catheterization and a myocardial biopsy suggested heart failure due to cardiac malignant lymphoma, and diastolic dysfunction was mild despite LVH. The patient underwent chemotherapy, and her cardiac function improved and was maintained.