Massive upper gastrointestinal bleeding is a life-threatening emergency which needs urgent intervention. Hematological malignancies are very rare causes of this type of bleeding and they usually originate from duodenum. In this case we present a gastric diffuse large B-cell lymphoma (DLBCL) causing massive upper gastrointestinal system bleeding. A 77-year-old male patient was admitted to emergency clinic with hematemesis and hematochezia. In physical examination patient was pale and sweaty; his vitals were unstable with a heart rate of 110 per minute and a blood pressure of 90/50mmHg. His hemoglobin level was found 7.5 g/dL and he was transfused with one unit of packed red blood cells. After his vitals were normalized, gastroscopy was performed showing mosaic pattern in corpus and antrum mucosa and multiple ulcers in various sizes, largest being approximately 2 cm in diameter, higher than mucosa covered with exude mostly on corpus and large curvature. Biopsy results were reported as DLBCL. Gastric mucosa is involved inmost of the DLBCL cases. Although not listed as a common cause of massive gastrointestinal bleeding DLBCL can cause life-threatening situations mostly because of its malignant nature.