Anti-PD-1/PD-L1 monoclonal antibodies result in a unique spectrum of side effects, widely known as immune-related adverse events. Toripalimab is an anti-PD-1 monoclonal antibody used for the treatment of some cancers. Here we report the first case, to our knowledge, of oral lichenoid drug reaction triggered by toripalimab. A 78-year-old man who was diagnosed with systemic metastatic prostate cancer presented with ulcers on the lower lip after the fifth cycle of toripalimab. We diagnosed him with oral lichenoid drug reaction based on clinical manifestation, histopathological findings and the history of anti-PD-1 therapy. The patient responded well to oral corticosteroids combined with helium-neon laser therapy. The anti-PD-1 therapy was not restarted because of stable disease, and the eruptions did not recur.