Immune checkpoint blockade therapy can induce immune-related toxicity, but cutaneous lymphoma development has not been reported. A 56-year-old woman presented with two well-demarcated erythematous macules on the right sole and vitiligo on her extremities. Her facial melanoma had been treated with combination therapy (ipilimumab and pembrolizumab), followed by pembrolizumab monotherapy, a year prior. Microscopy revealed small-to-medium-sized lymphocytes permeating along with the basal epidermal layer. These were immuno-positive for CD2, CD3, and CD5, and showed complete CD7 loss; CD30, TCR-beta F1, and PD-1 were also detected. They exclusively expressed CD8, not CD4, and had a Ki-67 labeling index of 30-40%. Epstein-Barr virus in-situ hybridization was negative. Clonal T-cell receptor beta and gamma chain gene rearrangements were detected. Hence, the lesions were diagnosed as mycosis fungoides. This is the first report of mycosis fungoides development after anti-melanoma immunotherapy. The patient is currently on steroid ointments and phototherapy.