Study of Pembrolizumab (MK-3475) in Adults With Recurrent/Metastatic Cutaneous Squamous Cell Carcinoma (cSCC) or Locally Advanced Unresectable cSCC (MK-3475-629/KEYNOTE-629)

Inclusion Criteria:

   - R/M cSCC cohort only:

   - Has cSCC that is either metastatic defined as disseminated disease, and/or
   unresectable disease that is not curable by surgery or radiation.

   - Has histologically-confirmed cSCC as the primary site of malignancy (metastatic skin
   involvement from another primary cancer or from an unknown primary cancer is not
   permitted).

   - LA cSCC cohort only:

   - Must be ineligible for surgical resection.

   - Participants who received prior radiation therapy (RT) to index site or must be deemed
   to be not eligible for RT.

   - Participants who received prior systemic therapy for curative intent are eligible
   regardless of regimen.

   - R/M cSCC cohort only:

   - Has metastatic disease defined as disseminated disease distant to the initial/primary
   site of diagnosis, and/or must have locally recurrent disease that has been previously
   treated (with either surgery or radiotherapy), and is not amenable to either curative
   surgery or radiotherapy.

   - Has measurable disease based on RECIST 1.1 as assessed by the central imaging vendor.

   - Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1 within
   10 days prior to the start of study treatment.

   - Has adequate organ function.

   - Has a tissue sample adequate for programmed death-ligand 1 (PD-L1) testing as
   determined by central laboratory testing prior to study allocation.

   - Has a life expectancy >3 months.

   - Female participants of childbearing potential must agree to use an adequate method of
   contraception during the study treatment period and for at least 120 days after the
   last dose of study treatment.

Exclusion Criteria:

   - Has cSCC that can be cured with surgical resection, radiotherapy, or with a
   combination of surgery and radiotherapy.

   - Has any other histologic type of skin cancer other than invasive squamous cell
   carcinoma as the primary disease under study, e.g. basal cell carcinoma that has not
   been definitively treated with surgery or radiation, Bowen's disease, Merkel cell
   carcinoma (MCC), melanoma.

   - Has had any prior allogeneic solid organ or bone marrow transplantation.

   - Has received prior therapy with an anti-programmed death protein-1 (anti-PD-1),
   anti-programmed death-ligand 1 (anti-PD-L1), or anti-PD-L2 agent or with an agent
   directed to another stimulatory or co-inhibitory T cell receptor (e.g. cytotoxic
   T-lymphocyte associated protein 4 [CTLA-4], Tumor necrosis factor receptor
   superfamily, member 4 [OX-40], tumor necrosis factor receptor superfamily member 9
   [CD137]).

   - Has received prior systemic anti-cancer therapy including investigational agents
   within 4 weeks prior to study allocation.

(Notes: Participants must have recovered from all AEs due to previously administered
therapies to ≤ Grade 1 or baseline. If a participant received major surgery, they must have
recovered adequately from the toxicity and/or complications from the intervention prior to
starting study treatment.)

   - Has received prior radiotherapy within 2 weeks of start 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 the first dose of
   study treatment.

   - Has known active central nervous system (CNS) metastases and/or carcinomatous
   meningitis.

   - Has an active autoimmune disease that has required systemic treatment in the past 2
   years (e.g. with use of disease-modifying agents, anticoagulants, corticosteroids or
   immunosuppressive drugs).

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

   - Has an active infection requiring systemic therapy.

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

   - Has a known history of Hepatitis B or known active Hepatitis C virus infection.

   - Is pregnant or breastfeeding or expecting to conceive or father children within the
   projected duration of the study, starting with the screening visit through 120 days
   after the last dose of study treatment.