Cutaneous adverse reactions to checkpoint inhibitor therapy commonly include non-specific dermatitis and pruritus, though the development of a wide range of dermatologic toxicities has been described, including lichenoid reactions. Occasional oral mucosal involvement has been reported, with rare erosive changes. Here, we present two cases of erosive oral lichen planus developing after PD-1 inhibitor therapy for malignant melanoma, with one case that later progressed to diffuse cutaneous involvement. Oral involvement in both cases was treated with topical high potency steroids and topical tacrolimus. This treatment regimen allowed one patient to continue immunotherapy and the other to resume immunotherapy after it was discontinued. These cases support erosive oral lichen planus as a notable, serious adverse effect of PD-1 inhibitors.