Key Clinical MessageSignet‐ring cell gastric carcinomas presenting as pericardial effusion early in diagnosis are rare and associated with high mortality and a poor prognosis. There are two interesting aspects of this case: primary gastric carcinoma presenting as cardiac tamponade and the metastatic behavior of gastric signet‐ring cell carcinoma.AbstractThis report describes an 83‐year‐old man diagnosed to have cardiac tamponade due to massive pericardial effusion. A cytological analysis of the pericardial effusion disclosed adenocarcinoma. The patient was treated with continuous pericardial drainage and the amount of pericardial effusion decreased.