Acquired isolated oculomotor nerve palsy (ONP) is a commonly encountered clinical entity in ophthalmology. While most cases are due to microvascular ischemia, the diagnosis of ONP requires careful evaluation for alternate life-threatening etiologies. We present a case of isolated complete pupil-involving ONP in a healthy 47-year-old man in whom aneurysmal compression was initially suspected. Investigations later revealed a diagnosis of neurosyphilis. Neurosyphilis is an extremely rare cause of isolated ONP and seldom reported in the literature. Timely recognition of this disease by ophthalmologists can help orient patients to the appropriate neurology and infectious disease services they need.