We report a case of primary gastric choriocarcinoma with liver metastasis and transverse colon invasion. A 51-year-old man presented with sudden syncope, and a gastroscopic biopsy showed gastric adenocarcinoma with recent bleeding. A palliative subtotal gastrectomy and segmental resection of the transverse colon was done, and formal histopathologic findings proved the tumor to be a choriocarcinoma with poorly differentiated adenocarcinoma. Choriocarcinoma is characterized by biphasic histologic patterns composed of syncytiotrophoblasts, cytotrophoblasts, and positive immunostaining for cytoplasmic hCG, as well as an increased serum hCG levels. This unusual tumor probably resulted from dedifferentiation of a primary adenocarcinoma. Therefore, in the case of a poorly differentiated adenocarcinoma of the stomach, meticulous examination to detect trophoblastic differentiation on tissue sections and evaluation of serum hCG should be made. This tumor is rapidly invasive and resistant to multiple chemotherapies.