Abstract Background Primary cardiac diffuse large B-cell lymphoma (CDLBCL) is an exceptionally rare entity, estimated to represent less than 1% of all primary cardiac tumours. In this case report, we emphasize the diagnostic importance of multimodality imaging and the need for additional procedures, such as tissue biopsy, in a case with a primary cardiac lymphoma presenting with cardiac tamponade. Case summary An 80-year-old male was admitted to the emergency department with a life-threatening tamponade demanding immediate sternotomy. Pre-operative echocardiography unveiled pericardial effusion and a thickened apex. While computed tomography ruled out an aortic dissection, surgery revealed an unexpected vascular-rich mass at the right ventricle and apex, too perilous for biopsy. Post-operative imaging misinterpreted this mass as a benign haematoma. Subsequently, the patient was admitted to the intensive care unit, but after a conservative treatment strategy, the patient died. An autopsy revealed a primary CDLBCL. Discussion This case demonstrates the deceptive nature of primary CDLBCL, often complicated by cardiac tamponade. It underscores the pivotal role of pathologic assessment, even amidst the perils of sternotomy, to determine the origin of abnormal cardiac masses. A heightened awareness among physicians is imperative, for such elusive diagnoses may slip by, with potentially fatal outcomes.