Functional neurologic disorders feature nervous system symptoms that cannot be explained by a neurological disease or other medical condition. The patient described here was a 21-year-old Japanese woman who was initially diagnosed with a functional neurologic disorder based on numbness and weakness of the limbs with no abnormalities in routine examinations. Further detailed examinations revealed monocytes in cerebrospinal fluid (CSF), and electroencephalography revealed widespread, low-voltage, slow waves with concentrated spindle waves. Thus, encephalitis was suspected, and steroid pulse therapy was initiated. Her symptoms subsequently improved. Afterward, CSF analysis was positive for serum anti-GQ1b IgG antibodies. We made a final diagnosis of Bickerstaff's brainstem encephalitis (BBE). Our report describes the difficult differentiation of functional neurologic disorders from BBE. Physicians and psychiatrists should be aware of BBE.