There have been few reports of the dual differentiation of different cell types within the same gastric tumor. Here, we report a rare case of poorly differentiated endocrine cell carcinoma with an associated differentiated signet ring cell population arising in the stomach. The histological appearance of the tumor by light microscopy matched the phenotype of endocrine cell carcinoma and signet ring cell differentiation with mucinous lakes. Cells with a phenotype intermediate between the two differentiated cell types were also seen in the tumor. Both the endocrine cell carcinoma and the signet ring cells were diffusely positive for chromogranin A and synaptophysin, a finding that is consistent with endocrine differentiation by immunohistochemical examination. The patient's postoperative clinical course had a poor prognosis, with aggressive tumor progression. Paraaortic lymph node recurrence was found 6 months after the operation, and the patient died of the primary disease 16 months after the surgical treatment.