We present a rare case of a pembrolizumab-induced Stevens–Johnson syndrome/toxic epidermal necrolysis (SJS/TEN). A 55-year-old woman with a history of metastatic cervical squamous cell carcinoma presented with a widespread mucocutaneous rash. Seventeen days after receiving her first cycle of pembrolizumab, she presented with fever, difficulty breathing, watery eyes, and painful oral ulcers. Physical examination revealed widespread erythematous papules and papulovesicles involving the trunk, upper extremity, and lower extremity and hemorrhagic plaques on the lower lip and buccal mucosa. Biopsy confirmed a diagnosis of SJS/TEN. This case highlights the importance of awareness of SJS/TEN as a possible adverse reaction for patients receiving pembrolizumab, a medication increasingly being used to treat metastatic or unresectable malignancies.