This phase I b study evaluated the safety and anti-tumor activity of pembrolizumab in Japanese patients with advanced melanoma. Pembrolizumab (2 mg/kg) was given every 3 weeks (Q3W) for up to 2 years or until confirmed progression or unacceptable toxicity. The tumor response was assessed as per the Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST v1.1) by both investigator review and central review. Forty-two patients with advanced melanoma received pembrolizumab. A primary cutaneous histology was observed in 34 patients (81.0%), while a primary mucosal histology was observed in 8 patients (19.0%). Thirty-four patients (81.0%) experienced treatment-related adverse events (AEs). The most common treatment-related AEs were pruritus, maculopapular rash, malaise, and hypothyroidism. Grade 3-5 treatment-related AEs occurred in 8 patients (19.0%). The only grade 3-5 treatment-related AE reported in at least two patients was anemia. There were two treatment-related deaths (unknown cause and cerebral hemorrhage). Among the 37 evaluable patients, the confirmed overall response rates (ORRs) determined by central review were 24.1% (95% CI 10.3-43.5) for cutaneous melanoma and 25.0% (95% CI 3.2-65.1) for mucosal melanoma. The responses were durable, and the median duration of response was not reached in either population. The median overall survival (OS) was not reached, with a 12-month OS of 82.7% for cutaneous melanoma and 51.4% for mucosal melanoma. The safety profile of pembrolizumab in Japanese patients was similar to that reported in the previous clinical studies. Pembrolizumab provided promising anti-tumor activity in Japanese patients with advanced melanoma.