We herein report a rare case of a massive upper gastrointestinal (GI) bleeding, caused by high-grade diffuse B-cell lymphoma of the duodenum, secondary to immunoproliferative small intestinal disease (IPSID) and treated with an emergency partial pancreatoduodenectomy. A 42-year-old man was admitted to our hospital because of hematemesis. Upper GI endoscopy was unrevealing because of the copious bleeding. Initially, the patient underwent conservative treatment, thus resulting in the temporary cessation of the bleeding. Later, the hemorrhage massively relapsed. An urgent abdominal ultrasound raised the suspicion of a large, possibly bleeding, neoplasm of the duodenum, which was finally confirmed by abdominal computed tomography. The patient underwent an emergency laparotomy, during which a partial pancreatoduodenectomy was performed (Whipple procedure). Histologically, the tumor was a high-grade B-cell lymphoma of the duodenum. The nearby small intestinal mucosa was suggestive of IPSID. A massive upper GI hemorrhage from a high-grade B-cell non-Hodgkin lymphoma of the duodenum, which develops secondary to IPSID, is a very rare clinical demonstration of this disease. Our case is one of the few reports in the English literature, for which the Whipple procedure has been performed as a curative treatment.