The therapeutic use of immunological modulators for cancer treatment is rapidly increasing. One of the immune checkpoint inhibitors, anti-PD-1/PD-L1 antibodies (pembrolizumab) is considered as a standard treatment for patients with advanced melanoma. Despite its clinical benefits, there are some concerns associated with immune-related adverse events. A 63-year-old male had been referred to our clinic due to developing skin problems during cancer treatment. He had been treated with pembrolizumab 2 times for malignant melanoma after foot amputation. He presented erythematous scaly patches on both hand and also showed nail involvement. Based on clinical and histological findings of psoriasis, we performed medication such as acitretin and topical ointment for treatment. He showed clinical improvement after the treatment. We report an interesting case of developing psoriasis after using pembrolizumab for malignant melanoma.