Fisher syndrome is a subtype of Guillain-Barré syndrome that is characterized by the three chief symptoms of acute-onset external ophthalmoplegia, ataxia, and loss of tendon reflexes. Herein, we report a case of Fisher syndrome encountered by us,with sphenoiditis as the antecedent infection. The patient was a 39-year-old man who visited a local doctor after developing the symptom of rhinorrhea, followed three days later by diplopia. Cranial MRI suggested sphenoid sinusitis, and the patient was referred to our hospital with suspected rhinogenous intraorbital complication. Neurological findings included bilateral abduction deficit, which was considered unlikely to be an ocular manifestation of an intraorbital complication of right unilateral sphenoiditis. In addition, the reduction of the tendon reflexes in the limbs and ataxia suggested the diagnosis of Fisher syndrome, which was then confirmed based on the albuminocytologic dissociation observed on cerebrospinal fluid examination and a positive blood test result for anti-GQ1b antibody. The symptoms were attributed to the production of antibodies against antigens expressed on the oculomotor, trochlear, abducens and other nerves (anti-GQ1b antibody) due to the antecedent infection. The present patient was thought to have developed Fisher syndrome following sphenoiditis as an antecedent infection. Care must be taken to differentiate this condition from rhinogenous intraorbital complication.