Herein we report a 57-year-old man with lower extremity edema and swelling in the scrotum who was found to have a periaortic soft tissue mass and hydronephrosis by computed tomography. With the most plausible diagnosis of retroperitoneal fibrosis, corticosteroid therapy was initiated; however, it did not improve his symptoms. Upper gastroscopy performed on day 20 post admission showed ulcerative regions with an irregular border and fusion of thickened rugae at the gastric angle; the diagnosis of gastric adenocarcinoma was confirmed histologically. It is important to always be aware of unrecognized malignancies that are accompanied by retroperitoneal fibrosis.