This study will assess whether post-surgery therapy with pembrolizumab improves recurrence-free survival (RFS) as compared to placebo for high-risk participants with melanoma (Stage IIIA \[\> 1 mm metastasis\], IIIB and IIIC). The study will also assess whether pembrolizumab improves RFS versus placebo in the subgroup of participants with programmed cell death-ligand 1 (PD-L1)-positive tumor expression. Participants will be stratified for stage of disease and region and then will be randomly assigned to receive either pembrolizumab or placebo as post-surgery therapy in Part 1. In Part 2, participants who experience a disease recurrence are eligible for pembrolizumab treatment (if treated with placebo in Part 1) or pembrolizumab rechallenge (if treated with pembrolizumab in Part 1).