Inclusion Criteria:

          -  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).

        Exclusion Criteria:

          -  A history of other malignancy with the exception of malignancies for which all
             treatment was completed at least 2 years before registration and the patient has no
             evidence of disease.

          -  Received radiation therapy within the area of interest.

          -  Currently receiving any other investigational agents.

          -  Metastatic disease.

          -  A history of allergic reactions attributed to compounds of similar chemical or
             biologic composition to pembrolizumab or other agents used in the study.

          -  Uncontrolled intercurrent illness including, but not limited to, ongoing or active
             infection, symptomatic congestive heart failure, unstable angina pectoris, or cardiac
             arrhythmia.

          -  Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any
             other form of immunosuppressive therapy within 7 days prior to the first dose of study
             treatment. The use of up to 10 mg/day of prednisone or equivalent is approved and does
             not exclude the patient from the trial.

          -  Active autoimmune disease that has required systemic treatment in past 2 years (i.e.,
             with use of disease modifying agents, > 10 mg of prednisone per day, or
             immunosuppressive drugs). Replacement therapy (e.g., thyroxine, insulin, or
             physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency,
             etc.) is not considered a form of systemic treatment. The use of up to 10 mg/day of
             prednisone or equivalent is approved and does not exclude the patient from the trial.

          -  Has a history of (non-infectious) pneumonitis/Interstitial lung disease that required
             maintenance steroids (>10 mg of prednisone) or current pneumonitis/interstitial lung
             disease.

          -  Has received a live vaccine or live-attenuated vaccination within 30 days of planned
             treatment start. Administration of killed vaccines is allowed.

          -  Pregnant and/or breastfeeding. Women of childbearing potential must have a negative
             pregnancy test within 14 days of study entry.

          -  Patients with HIV are eligible unless their CD4+ T-cell counts are < 350 cells/mcL or
             they have a history of AIDS-defining opportunistic infection within the 12 months
             prior to registration. Concurrent treatment with effective ART according to DHHS
             treatment guidelines is recommended.