Sinonasal neuroendocrine carcinomas (NECs) are rare tumors. We present a rare case of intracranial invasion of sinonasal small-cell NEC. A 61-year-old woman with nasal obstruction and bleeding was referred to our hospital. Computed tomography showed a polyp-like tumor occupying her left nasal cavity and extending to the paranasal sinuses and anterior cranial fossa. The tumor was removed using a transfacial approach by otolaryngologists and a bifrontal cranial approach by neurosurgeons. In histopathological analyses, we found that the tumor presented with both an epithelial and neuroendocrine nature, and was diagnosed as a small-cell NEC. Post-surgery, she received localized radiation therapy and chemotherapy, and is alive, 18 months after diagnosis. In cases where it is difficult to perform a differential diagnosis of tumors arising from the frontal cranial base and extending to the nasal and cranial sides, NEC should be considered as a possibility.