Inclusion Criteria:

          -  Has a diagnosis of HCC by radiological criteria and/or pathological confirmation.

          -  Has an eligibility scan (CT of the chest, triphasic CT scan or MRI of the abdomen, and
             CT or MRI of the pelvis) confirming complete radiological response ≥4 weeks after
             complete surgical resection or local ablation. Randomization needs to occur within 12
             weeks of the date of surgical resection or local ablation.

          -  Has no radiologic evidence of disease prior to enrollment.

          -  Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 within 7
             days prior to Cycle 1, Day 1.

          -  Has a Child-Pugh class A liver score (5 to 6 points) within 7 days prior to Cycle 1,
             Day 1.

          -  Has alpha fetoprotein (AFP) concentration lower than 400 ng/mL within 28 days prior to
             Cycle 1, Day 1.

          -  Has controlled hepatitis B (Hep B).

          -  Has recovered adequately from toxicity and/or complications from the local
             intervention (surgical resection or local ablation) prior to starting study treatment.

          -  If female, is not pregnant or breastfeeding, and at least one of the following
             conditions applies: 1) Is not a woman of childbearing potential (WOCBP); or 2) Is a
             WOCBP and using a contraceptive method that is highly effective or be abstinent from
             heterosexual intercourse as their preferred and usual lifestyle (a WOCBP must have a
             negative pregnancy test within 72 hours before the first dose of study treatment).

          -  If undergoing surgical resection, has submitted a tumor tissue sample during
             Screening.

          -  Has adequate organ function.

        Exclusion Criteria:

          -  Has a known additional malignancy that is progressing or has required active
             antineoplastic treatment (including hormonal) or surgery within the past 3 years.

          -  Has had esophageal or gastric variceal bleeding within the last 6 months.

          -  Has clinically apparent ascites on physical examination.

          -  Has had clinically diagnosed hepatic encephalopathy in the last 6 months.

          -  Has received local therapy to liver ablation other than with radiofrequency or
             microwave ablation.

          -  Has a history of (noninfectious) pneumonitis that required steroids or has current
             pneumonitis.

          -  Has an active infection requiring systemic therapy.

          -  Has dual active Hepatitis B Virus (HBV) and Hepatitis C Virus (HCV) infection at study
             entry.

          -  Has a known history of human immunodeficiency virus (HIV) infection.

          -  Has known active tuberculosis (TB; Bacillus tuberculosis).

          -  Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti PD-L2 agent or with
             an agent directed to another stimulatory or co-inhibitory T-cell receptor (eg, CTLA-4,
             OX-40, CD137).

          -  Has received prior systemic anti-cancer therapy for HCC including investigational
             agents.

          -  Is receiving any of the following prohibited concomitant therapies:1) Antineoplastic
             systemic chemotherapy or biological therapy; 2) Immunotherapy not specified in this
             protocol; 3) Investigational agents other than pembrolizumab; 4) Radiation therapy; 5)
             Oncological surgical therapy; or systemic glucocorticoids for any purpose other than
             to modulate symptoms from an AE that is suspected to have an immunologic etiology.

          -  Has received a live vaccine within 30 days prior to the first dose of study treatment.

          -  Is currently participating in or has participated in a study of an investigational
             agent or has used an investigational device within 4 weeks prior to Cycle 1, Day 1.

          -  Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy
             or any other form of immunosuppressive therapy within 7 days prior to Cycle 1, Day 1.

          -  Has severe hypersensitivity (≥Grade 3) to pembrolizumab and/or any of its excipients.

          -  Has an active autoimmune disease that has required systemic treatment in past 2 years.

          -  Has a known psychiatric or substance abuse disorder that would interfere with the
             participant's ability to cooperate with the requirements of the study.

          -  Has had an allogenic tissue/solid organ transplant.