A 37-year-old woman presented with isolated right-sided oculomotor nerve palsy. Neurologic examination revealed no other disorder. Computed tomography of the paranasal sinuses demonstrated complete opacification of the sphenoid sinus. Dense mucoid fluid was drained from the sphenoid sinus via an endoscopic transseptal sphenoidotomy. A biopsy confirmed the diagnosis of sphenoid sinus mucocele. At follow-up 4 weeks postoperatively, the patient's ocular symptoms were markedly alleviated. Considering rare causes of isolated oculomotor nerve palsy, such as sphenoid sinus mucocele, is important in the differential diagnosis, even in patients with well-known risk factors such as diabetes mellitus.