Gastric cancer is the third most common malignancy among gastrointestinal malignancies. With the advance of new treatments, overall survival in gastric cancer is extending, and metastasis to atypical sites is seen more commonly. Leptomeningeal metastasis is one such atypical metastasis for gastric cancer. We report a case of gastric adenocarcinoma with leptomeningeal metastasis as an atypical involvement. A 39-year-old female, presenting with headache, vertigo, horizontal gaze palsy, visual disturbances, and seizures, was admitted to our hospital in August 2009. The funduscopic examination revealed the presence of bilateral papilledema. Magnetic resonance imaging of the brain showed diffuse leptomeningeal enhancement and biventricular dilatation. Cytological examination of the cerebrospinal fluid revealed malignant cells. These findings were consistent with leptomeningeal carcinomatosis. Six months before, she was diagnosed as having gastric cancer by upper gastrointestinal tract endoscopy, which was performed as a part of the diagnostic work-up to clarify the cause of her abdominal ascites. She received six cycles of docetaxel-cisplatin-5-fluorouracil for metastatic gastric cancer, and she developed the above-mentioned symptoms under chemotherapy. She was included in a craniospinal radiotherapy program and received intrathecal methotrexate treatment. We present this case report since leptomeningeal carcinomatosis of gastric cancers is a rare clinical entity, and treatment strategies remain challenging for clinicians.