Here, we report a case of syphilis presented with optic neuritis to consider neurosyphilis as one of the differential diagnoses of optic neuritis. A 25-year-old male attended at outpatient department of chittagong eye infirmary and training complex institute with a history of a sudden loss of vision in the left eye for 20 days. On eye examination, the patient had reduced visual acuity on the left eye (6/60), and the left pupil showed a relative afferent pupillary defect and swollen left optic disc. No other abnormalities were found in a routine blood test and magnetic resonance imaging of the brain. Intravenous corticosteroid was administered for 3 days followed by oral corticosteroid. His vision was gradually improving within a month and became 6/9 in the left eye, but after a month, the patient returned with the blurring of vision in the same eye for 3 days. An extensive serum biochemical and serological test and cerebrospinal fluid (CSF) analysis was done including syphilis serology and human immunodeficiency virus (HIV) serology. Venereal disease research laboratory (VDRL) test and Treponema pallidum hemagglutination assay (TPHA) were found positive with high titer (1:1280) and rapid plasma reagin (RPR) titer of 1:64 in blood. The CSF analysis showed leukocytosis, and VDRL and TPHA were also found positive with high RPR titer. The HIV serology test was negative. The patient was treated with injectable ceftriaxone 2 g intravenously for 14 days and also injectable corticosteroid. His vision was improved within this period. Unilateral optic neuritis due to syphilis without other ocular features is uncommon but should be considered if a patient presents with visual loss and optic disc swelling. Early diagnosis based on clinical suspicion and prompt management is important to prevent visual impairment and subsequent neurological complications.