Leptomeningeal disease, also known as leptomeningeal carcinomatosis, occurs when cancer metastasizes to the meninges. This rare complication is associated with a poor prognosis. It is most commonly seen in patients with metastatic breast cancer, lung cancer, and melanoma. However, it is extremely rare in patients with metastatic gastric cancer. A 64-year-old female with poorly differentiated gastric adenocarcinoma metastatic to the peritoneum developed new neurological symptoms twelve months after initiating palliative chemotherapy. Her uptrending tumor markers, brain magnetic resonance imaging (MRI) findings, and lumbar puncture results were consistent with leptomeningeal disease. The patient was started on treatment with intrathecal methotrexate (IT MTX), which resulted in significant improvement in her neurological symptoms. Leptomeningeal disease in gastric cancer has limited treatment options due to poor blood-brain barrier penetration. IT MTX is a potentially effective treatment for patients with leptomeningeal disease from gastric cancer.