We report a case of unilateral Keros type III variant of the cribriform plate (olfactory fossa >7 mm deep) that caused non-traumatic pneumocephalus. A 61-year-old male patient presented to the emergency room complaining of acute-onset headache after blowing his nose. Computed tomographic (CT) examination showed a massive pneumocephalus; a CT scan from three months before showed a Keros type III variant. The defect was repaired endoscopically with mucoperichondrial and mucoperiostal grafts. Pneumocephalus that has developed in a Keros type III variant has been theorised about, but never reported to our knowledge. Radiologists should be familiar with the Keros classification so that they can identify patients who are at increased risk of fractures of the cribriform plate.