• Histologically or cytologically confirmed mucosal melanoma that has undergone surgical resection. Patient must not have received prior radiation therapy within the area of interest.

• At least 16 years of age.

• Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2 (Karnofsky ≥ 70%)

• Normal bone marrow and organ function as defined below:

‣ Absolute neutrophil count ≥ 1,200/mcL

⁃ Platelets ≥ 100,000/mcL

⁃ Total bilirubin ≤ 1.5 x institutional upper limit of normal (IULN)

⁃ AST(SGOT)/ALT(SGPT) ≤ 3.0 x IULN

⁃ Creatinine clearance \> 30 mL/min by Cockcroft-Gault

• The effects of pembrolizumab on the developing human fetus are unknown. For this reason, women of childbearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control, abstinence) prior to study entry, for the duration of study participation, and 6 months after last dose of pembrolizumab. Should a woman become pregnant or suspect she is pregnant while participating in this study, she must inform her treating physician immediately. Men treated or enrolled on this protocol must also agree to use adequate contraception prior to the study, for the duration of the study, and 6 months after last dose of pembrolizumab.

• Ability to understand and willingness to sign an IRB approved written informed consent document (or that of legally authorized representative, if applicable).