Small cell carcinoma of the stomach, which is considered synonymous with neuroendocrine carcinoma, is rare. We encountered a case of mixed adenoneuroendocrine carcinoma. Gastrointestinal endoscopy revealed a type 0 IIa + IIc lesion in the stomach. Biopsy suggested well-differentiated adenocarcinoma. Endoscopic submucosal dissection was performed and gastric cancer comprising both well to moderately differentiated adenocarcinoma within the mucous membrane and small cell carcinoma invading to the submucosa with accompanying vessel invasion was found. Mixed adenoneuroendocrine carcinoma was diagnosed and the small cell carcinoma was thyroid transcription factor-1 positive. No tumor lesions were apparent in other organs, so distal gastrectomy and lymph node dissection were performed. Lymph node metastases were not observed. The patient is currently alive without recurrence. Thyroid transcription factor-1-positive gastric neuroendocrine carcinoma is very rare, and the thyroid transcription factor-1-positive rate may contribute to the prediction of prognosis and treatment selection. Accumulation of cases is thus very important.