We describe a patient with mesial temporal T2-weighted image hyperintensity on magnetic resonance imaging that mimicked paraneoplastic limbic encephalitis. The patient showed pupillary abnormalities suggestive of a diagnosis of neurosyphilis, and the diagnosis was supported by the results of a serum Treponema pallidum hemagglutination assay (TPHA) and cerebrospinal fluid examination. Making a diagnosis of neurosyphilis is occasionally difficult because of the variety of clinical and imaging findings. Appropriate diagnosis and commencing adequate treatment are needed for a good prognosis; thus, neurosyphilis should be included in the differential diagnosis of mesiotemporal magnetic resonance imaging abnormalities.