Carcinoma arising from Rokitansky–Aschoff sinus (RAS) is extremely rare; only eight cases have been reported in the literature. Herein is reported a case of minute adenocarcinoma arising in RAS. A 77-year-old Japanese man with gallbladder stones underwent cholecystectomy. A tiny submucosal tumor (1 cm × 1 cm) was incidentally recognized. Histologically, the submucosal tumor was located in the subserosa and, to a lesser extent, in the fibromuscular layer. It was adenocarcinoma. RAS were recognized within the tumor, and there was a gradual transition between RAS and the adenocarcinoma. Mucin histochemistry indicated neutral and acidic mucins in the cytoplasm and lumens of the adenocarcinoma cells. Immunohistochemistry showed that the adenocarcinoma cells were positive for cytokeratin, epithelial membrane antigen, carbohydrate antigen 19-9, K-i67 (labeling = 80%), MUC1, MUC5AC and MUC6. In contrast, the adenocarcinoma cells were negative for CEA, c-erbB2, p53 protein, MUC2 and CD10. In summary, minute subserosal adenocarcinoma, which arose in RAS, was found incidentally; therefore careful examination of resected gallbladders is necessary.