Background: Both primary pancreatic lymphoma (PPL) and primary splenic lymphoma (PSL) represent rare entities. PPL typically arises in the head of the pancreas but may arise in other locations also. PSL usually presents with nonspecific symptoms, including left upper quadrant pain, weight loss, and fever. This report describes a patient with a large left upper quadrant mass, which initially was believed to be a primary pancreatic mass, but which on final pathology appeared to be consistent with a PSL. Presentation: The patient is a 64-year-old woman who initially presented with symptoms of left upper quadrant abdominal pain and distension; she subsequently was found to have an 18 cm heterogeneous mass arising from the pancreatic tail. She underwent a distal pancreatectomy with splenectomy. Final pathology confirmed a diffuse large B cell lymphoma arising from the splenic parenchyma. Conclusions: Both PPL and PSL are rare causes of left upper quadrant masses. In this case, we describe a large lymphoma that appeared to arise from the tail of the pancreas, but on final pathology was found to be splenic in origin. Differentiating these two clinical entities is important for prognostication and treatment. A multimodal approach with surgical resection followed by chemotherapy is preferred.