We report the case of a 71-year-old man affected by testicular large B-cell lymphoma (DLBCL), treated with right orchiectomy and first-line chemotherapy (R-CHOP, 8 cycles). A complete remission was obtained after therapy. Twenty-two months after the primary diagnosis the patient suddenly presented dyspnoea and superior vena cava syndrome; thus, he underwent a CT scan that revealed a large mass in the right atrium, expanding to the superior vena cava. A differential diagnosis between a neoplastic mass and a clot was proposed. The subsequent MR did not clarify the nature of the mass; therefore, the patient underwent an 18F-FDG PET/CT scan (PET/CT), after a specific preparation to reduce fluoro-deoxyglucose (FDG) myocardial uptake. PET/CT revealed an intense FDG uptake involving the whole mass (SUVmax 9.4), suggestive for neoplasm and confirmed by the subsequent endocardiac biopsy. The patient was treated with 8 cycles of R-COMP, obtaining a complete remission, as indicated by the PET/CT performed after the seventh cycle of therapy. The case that we are reporting highlights that DLBCL can have an uncommon relapse presentation in the atrium. PET/CT, compared to conventional imaging, can be a valuable tool to detect early and better characterize cardiac lesions in order to improve the poor prognosis of these conditions.