Primary cardiac lymphomas (PCLs) are extremely rare and affect the heart. Patients with PCLs usually have delayed diagnosis and treatment. As a consequence, their prognosis is quite unfavorable, and their median survival is approximately 7 months. Herein, we report a 64-year-old man who underwent liver transplantation, presented with chest pain and exertional dyspnea, developed a huge cardiac mass within 2 months and passed away on day 3 of hospitalization. Histological examination revealed diffuse large B-cell lymphoma (DLBCL), which is a rare cardiac tumor with a poor prognosis. In this case, DLBCL was only detected postmortem. The extension of the mass and its relationship with the heart were explored with non-invasive cardiac imaging. Despite the rarity of DLBCL, it should be considered in the differential diagnosis of cardiac tumors.