Background: Sinonasal carcinoma is a rare and aggressive malignancy, with leptomeningeal carcinomatosis being an uncommon presentation. Case Report: We report a rare case of leptomeningeal carcinomatosis secondary to undifferentiated sinonasal carcinoma in a patient presenting with paraparesis. CSF analysis revealed atypical cells, and MRI demonstrated diffuse nodular leptomeningeal enhancement. Conclusion: A high index of suspicion is essential for diagnosing leptomeningeal carcinomatosis in patients with sinonasal carcinoma presenting with neurological deficits.