Pembrolizumab is an anti-PD-1 humanized monoclonal antibody, standardized therapy for the treatment of metastatic melanoma, providing an overall survival benefit to patients; however, it may be associated with immune-related adverse effects (irAE). We describe the case of a patient with a history of metastatic melanoma undergoing treatment with pembrolizumab, who presented visual acuity deterioration due to inflammatory involvement of the optic nerves. There are few reported cases of an optic neuritis side effect of treatment with immune checkpoint inhibitors. The patient received high doses of steroids and plasmapheresis with improved light perception. This case highlights the need to recognize atypical processes, mediated by the immune system, associated with treatment with immune checkpoint inhibitors (ICI), including pembrolizumab.