The diagnosis of leptomeningeal metastases is sometimes difficult when the cytology of cerebrospinal fluid is negative. We report a rare case of leptomeningeal metastases that required differentiation from paraneoplastic limbic encephalitis. A 67-year-old man with extensive-stage small cell lung cancer was admitted for a sudden decrease in the level of consciousness. He suffered memory disturbances that began the day before admission. Diffusion-weighted and fluid-attenuated inversion recovery images of brain magnetic resonance imaging (MRI) showed bilateral symmetric areas of hyperintensity in the hippocampus, amygdala, insular cortex, and medial temporal lobe; contrast enhancement was positive. Cytology of the cerebrospinal fluid (CSF) was negative. Anti-N-methyl-d-aspartate receptor antibody and herpes simplex virus DNA were not detected in the CSF. Paraneoplastic Limbic encephalitis was suspected due to his symptoms and brain MRI scan. The patient developed generalized seizures after admission. High-dose methylprednisolone and intravenous immune globulin were administered, but his condition did not improve. Uncontrollable seizures persisted and he died in the hospital at day 13. Autopsy revealed leptomeningeal metastasis and invasion of cancer cells into the limbic system. Contrast-enhanced MRI should be performed even if limbic encephalitis is suspected, and leptomeningeal metastases should be suspected if the lesions are enhanced.