This study aimed to report an unusual presentation of an advanced gastric adenocarcinoma. Leptomeningeal carcinomatosis is a rare event in gastric adenocarcinoma. It is much more uncommon as the primary manifestation in post-mortem evaluation of the cause of death in a patient presenting with headache and neurological signs and symptoms. Herein, we discuss our experience with a case of gastric adenocarcinoma, who was diagnosed after death, presenting with neurological signs and symptoms of leptomeningeal carcinomatosis. A 52-year-old gentleman presented with intractable headache and neck pain as well as vertigo. His physical examination showed only decreased deep tendon reflexes. He died after a short period of coma. Post-mortem evaluation showed numerous signet ring cells in the subarachnoid space as well as gastric malignant ulcer. In patients with intractable headache with no identifiable cause, meningeal involvement and infiltration should be considered as the probable underlying cause. Radiologic findings are not significant; however, lumbar puncture can be diagnostic.