Cemiplimab is a novel programmed death-1 inhibitor recently approved for advanced cutaneous squamous cell carcinoma. Immune-related adverse events derived from cemiplimab are similar to other anti-PD-1 drugs, including gastrointestinal and cutaneous toxicities. Oral immune-related adverse events were not reported with cemiplimab in previous studies; thus this case report warns of the fact that the oral cavity may be a site of immune-related adverse events during programmed death-1 block therapy and that this can lead to significant limitations when not properly treated. The present report describes the case of a patient with locally advanced cutaneous squamous cell carcinoma metastatic to cervical lymph nodes who developed dysphagia due to large and painful oral ulcers after a single dose of cemiplimab. The patient also exhibited a sarcoid-like reaction in mediastinal lymph nodes. No immune-related adverse events were found in any other organs. The oral lesions showed significant improvement after topical and short-course systemic corticosteroids, and low-level laser therapy was also performed in the oral lesions. The patient achieved a near-complete response and treatment was discontinued. This article discusses in detail the clinical outcomes and oral toxicity management of cemiplimab therapy for cutaneous squamous cell carcinoma.