This 2-part study will evaluate the safety and efficacy of pembrolizumab (MK-3475) compared to placebo in participants with surgically resected high-risk Stage II melanoma. Participants in Part 1 will receive either pembrolizumab or placebo in a double-blind design every 3 weeks (Q3W) for up to 17 cycles/\~1 year (each cycle = 21 days). Participants who complete the initial treatment of 17 cycles of pembrolizumab in Part 1 and experience disease recurrence may be eligible for re-challenge with pembrolizumab at the same dose and schedule of 200 mg Q3W (21-day cycles) for up to 35 cycles (up to \~2 years) in Part 2 in an open label design. Participants who complete the initial treatment of placebo and experience disease recurrence may be eligible to switch over to pembrolizumab 200 mg Q3W (21-day cycles) for up to 35 cycles (up to \~2 years) in Part 2 in an open label design. The primary hypothesis of this study is that pembrolizumab increases recurrence-free survival (RFS) compared to placebo. Per protocol, response/ progression or adverse events (AEs) during re-challenge/switch-over in Part 2 will not be counted towards the RFS outcome measure or safety outcome measures respectively.