A gastric adenocarcinoma is the second most common cause of cancer deaths worldwide, but there are some geographical differences in its incidence. A gastrointestinal tumor is an uncommon disease with a wide spectrum of aggressive behavior. These two tumors have a distinct pathogenesis, and synchronous occurrence of an adenocarcinoma and a GIST (gastrointestinal stromal tumor) in the stomach is very rare. We report a case of synchronous occurrence of a gastric adenocarcinoma and GIST in a 64-year-old man. We performed the following tests: barium swallowing test, gastroduodenoscopy, and CT scanning. We performed a total gastrectomy, and a 9 cm-sized GIST at the fundus and a small early gastric cancer at the antrum were confirmed pathologically. Some explanations for this rare case exist, but in our opinion, the synchronous occurrence is a rare but probable event that can happen in an endemic area. We describe a case of synchronous occurrence of a gastric adenocarcinoma and GIST in the stomach, with a review of the literature.