Optic neuritis could lead to severe visual impairment despite corticosteroids. Our aim was to evaluate the rate of visual improvement in patients treated with plasma exchange (PLEX) for severe steroid unresponsive optic neuritis and to identify predictive factors of outcome. Thirty-four patients (41 optic nerves damaged) with remaining visual acuity of 0.1 or less despite steroid pulse therapy were treated with PLEX from September 2010 to May 2015. Demographic and clinical neuro-ophthalmic findings, and spectral domain-optical coherence tomography data before PLEX treatment were analyzed. The mean symptom duration before PLEX was 34.6 days (median 28 days; range 6–92 days). After PLEX, the median final visual acuity was 0.8 and in 56 % of cases, final acuity was 0.5 or better. Past history of ipsilateral optic neuritis was associated significantly with poor outcome defined as final acuity less than 0.5. No significant difference in the visual outcome after PLEX was found between multiple sclerosis and neuromyelitis optica. In conclusion, this observational study showed that PLEX as second-line therapy led to a functionally important visual recovery in more than half patients with severe optic neuritis.