A 15-year-old girl with neurosyphilis was misdiagnosed as having viral encephalitis with psychotic symptoms and Parkinsonism. We found that she was experiencing visual hallucinations, persecutory delusions, flattening of affect, poorness of thought, tremors, four-limb rigidity, and restlessness, and she was unable to communicate with others. The Venereal Disease Research Laboratory serum test and further lumbar puncture enabled us to diagnose her with neurosyphilis. After antibiotic treatment, her psychotic symptoms and Parkinsonism were relieved. From this case, we believe that it is important to keep organic psychosis in mind during the diagnostic workup, and we argue that routine syphilis screening is necessary in psychiatry clinical practice.