Background: Pembrolizumab (pembro) is a monoclonal antibody against PD-1 designed to block its interaction with PD-L1 and PD-L2 and permit an antitumor immune response. In KEYNOTE-012, pembro showed a 22% ORR (RECIST v1.1, central review) and a manageable safety profile in patients (pts) with advanced gastric cancer. The randomized, phase 3 KEYNOTE-062 study (NCT02494583) is designed to compare the efficacy and safety of pembro alone or in combination with cisplatin + a fluoropyrimidine with those of cisplatin + a fluoropyrimidine as first-line therapy for PD-L1⁺/HER2^– advanced gastric or GEJ adenocarcinoma. Methods: Key eligibility criteria include age ≥ 18 y, locally advanced or metastatic PD-L1⁺/HER2^–gastric or GEJ adenocarcinoma, ECOG PS 0-1, no active autoimmune disease or brain metastases, and no prior therapy for advanced disease. Pts are randomized 1:1:1 to pembro 200 mg Q3W (arm 1), pembro + cisplatin 80 mg/m² Q3W + 5-fluorouracil (5-FU) 800 mg/m² on days 1-5 of each Q3W cycle (arm 2), or placebo Q3W + cisplatin + 5-FU (arm 3); 5-FU may be replaced with capecitabine 1000 mg/m² twice daily on days 1-14 of each cycle. Randomization is stratified by region (Europe/North America/Australia vs Asia vs rest of world), disease status (locally advanced vs metastatic), and chosen fluoropyrimidine (5-FU vs capecitabine). Arm 1 is open label; in arms 2 and 3, assignment to pembro vs placebo is double blind. In all arms, treatment will continue for 35 cycles or until progressive disease, unacceptable toxicity, or pt/investigator decision. Response will be evaluated every 6 wk per RECIST v1.1 by central review and per RECIST adapted for immunotherapy response patterns; eligible pts may continue treatment beyond initial RECIST-defined progression. AEs will be assessed throughout treatment and for 30 d thereafter (90 d for serious AEs) and graded per NCI CTCAE v4.0. Pts will be followed for survival every 3 mo. OS and PFS per RECIST v1.1 are the primary study end points; secondary end points include ORR and duration of response. Enrollment in KEYNOTE-062 is ongoing and will continue until ~750 pts have enrolled. Clinical trial information: NCT02494583