Large cell neuroendocrine carcinoma (LCNEC) is a rare malignant tumor that typically arises in the lungs. It is especially rare in the sinonasal cavity, and treatment has not been established. In this study, we present the case of a 56-year-old woman with a large sinonasal LCNEC that extended into her brain. We performed endonasal endoscopic and transcranial combined surgery followed by chemoradiation therapy. The combined surgery enabled us to approach and remove the extensive tumor from two different directions at one time less invasively. We have achieved good tumor control for 18 months so far.