We report a rare case of adenocarcinoma arising in a gastric partial diverticulum in the upper portion of the stomach. The lesion had been followed up for approximately 14 years as a gastric submucosal tumor. However, a recent regular check-up revealed mucous material with some neoplastic tissue discharged from the top of the lesion. A surgically resected specimen showed a well-demarcated submucosal lesion identical to a pseudo-diverticulum carrying a distinctive intramucosal minimally invasive adenocarcinoma in part, with surrounding non-neoplastic but hyperplastic mucosal components. Intestinal phenotype, along with gastric foveolar, pyloric gland-type phenotypes, and neoplastic cells with neuroendocrine differentiation, were also identified in the adenocarcinoma. Chronic and persistent irritation within the diverticulum was postulated to be implicated in the carcinogenesis of the lesion, which carried no definite Helicobacter pylori microorganisms. We believe it is crucial not to overlook carcinoma in a diverticulum presenting as a long-standing submucosal tumor.