Primary cardiac lymphoma (PCL) is an extremely rare disorder. In this report, a 57-year-old male with diffuse large B-cell lymphoma involving the heart and great vessels is presented. Trans-thoracic echocardiography was the first modality used to establish the diagnosis. Fluorine-18-fluorodeoxyglucose positron emission tomography/computed tomography (FDG PET/CT) showed diffuse increased metabolic activity of the heart walls and hypermetabolic lesions occupying cardiac chambers in some areas. The patient underwent systemic chemotherapy, and after 13 days, a marked regression of the tumour mass was evident based on echocardiographic examination. After completing six R-CHOP chemotherapy treatments, PET imaging was planned to control the residual mass, but the patient was intubated due to pneumonia that developed after the sixth chemotherapy session and subsequently died due to sepsis.