The FDA approved pembrolizumab on June 29, 2020, for the treatment of patients with unresectable or metastatic microsatellite instability-high (MSI-H) colorectal cancer with no prior systemic treatment for advanced disease. The approval was based on data from Study Keynote-177, which randomly allocated patients to receive either pembrolizumab or standard of care (SOC) with chemotherapy. Overall survival (OS) and independently assessed progression-free survival (PFS) were the primary endpoints. At the time of the final PFS analysis and second prespecified interim OS analysis, the estimated median PFS was 16.5 months (95% CI: 5.4–32.4) versus 8.2 months (95% CI: 6.1–10.2) in the pembrolizumab and SOC arms, respectively [HR: 0.60 (95% CI: 0.45–0.80); two-sided P = 0.0004]. FDA assessed unblinded OS data during the review of the application and identified no safety concerns that would preclude approval of this supplement. Adverse reactions occurring in >30% of patients receiving pembrolizumab were diarrhea, fatigue/asthenia, and nausea. Adverse reactions occurring in >30% of patients receiving SOC were diarrhea, nausea, fatigue/asthenia, neutropenia, decreased appetite, peripheral neuropathy (high-level term), vomiting, abdominal pain, constipation, and stomatitis. Duration of treatment in the pembrolizumab arm was almost double (median 11.1 months, range 0–30.6 months) than the duration of treatment in patients receiving SOC (median, 5.7 months). Approval of pembrolizumab is likely to change the treatment paradigm for first-line treatment with MSI-H advanced colorectal cancer given the study results and different safety profile.