Background: Adjuvant pembrolizumab showed significantly longer recurrence-free survival compared with placebo in resected stage III melanoma in the KEYNOTE-054 study [1]. KEYNOTE-716 is a randomized, placebo-controlled, multicenter phase 3 study of adjuvant pembrolizumab in patients with surgically resected high-risk stage II melanoma. Methods: Patients must be ≥12 years of age and have newly diagnosed, completely resected stage IIB/IIC cutaneous melanoma, defined by the AJCC Cancer Staging Manual, 8th edition [2] (wide excision and negative sentinel lymph node biopsy, with no evidence of distant metastasis). Patients cannot have mucosal or uveal melanoma or have received prior treatment for melanoma, including radiation, beyond resection of primary disease within 12 weeks of the start of study therapy. The study has a 2-part design. In the double-blind phase (part 1), patients will be randomly assigned 1:1 to receive pembrolizumab 200 mg for patients ≥18 years or 2 mg/kg for patients 12-17 years (maximum dose, 200 mg) or placebo every 3 weeks for 17 cycles. Stratification: 1 stratum for pediatric patients (12-17 years); 3 strata for adult patients per T stage (T3b/T4a/T4b). Study treatment will begin within 12 weeks of complete resection. Tumor imaging will be performed every 24 weeks while treatment is ongoing, at the end of treatment, every 6 months for the first 3 years off treatment, and then yearly for up to 2 years or until recurrence (up to 5 years of total imaging). Adverse events will be graded per NCI Common Terminology Criteria for Adverse Events, version 4.0. In the unblinded phase (part 2), patients with confirmed recurrence may be rechallenged (patients received pembrolizumab in part 1) or crossed over to pembrolizumab (patients received placebo in part 1). Resected local or distant recurrence or unresectable disease will be treated for an additional 17 or 35 cycles, respectively. Tumor imaging in part 2 will occur every 12 weeks while treatment is ongoing. The primary end point is recurrence-free survival; secondary end points are distant metastasis-free survival, overall survival, and safety. Approximately 954 patients will be enrolled. Clinical trial information: NCT03553836