Background: Gastric cancer is the fourth leading cause of cancer death worldwide. Capecitabine + oxaliplatin (CAPOX) is a standard first-line treatment for advanced gastric cancer. Claudin (CLDN)18.2 has emerged as a promising targetable biomarker. In healthy tissue, CLDN18.2, a tight junction protein, is confined to gastric mucosa (ie, cells in the pit and base regions of gastric glands). Upon malignant transformation, structural loss in gastric or gastroesophageal junction (G/GEJ) adenocarcinoma cells may allow antibodies more access to previously unavailable CLDN18.2. Zolbetuximab is a chimeric IgG1 monoclonal antibody that specifically binds to CLDN18.2 and mediates cell death through antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity. Results of a phase 2 study (NCT01630083) showed prolonged survival of patients with CLDN18.2-positive (CLDN18.2⁺) advanced G/GEJ adenocarcinoma treated with zolbetuximab + epirubicin, oxaliplatin, and capecitabine (EOX) vs EOX alone. Methods: This phase 3, double-blind, placebo-controlled study (NCT03653507) will enroll ~500 adult patients from global sites. Patients are required to have CLDN18.2⁺/HER2⁻ locally advanced unresectable or metastatic G or GEJ adenocarcinoma that is radiographically evaluable per RECIST v1.1. Patients are not permitted to have received prior treatment with chemotherapy for advanced or metastatic G or GEJ adenocarcinoma. Patients will be randomly assigned 1:1 to receive either zolbetuximab plus CAPOX or placebo plus CAPOX. Randomization will be stratified by region (Asia vs non-Asia), number of metastatic sites (0 to 2 vs ≥3), and prior gastrectomy (yes vs no). Zolbetuximab will be administered at a loading dose of 800 mg/m² IV on Cycle 1 Day 1 followed by 600 mg/m² IV every 3 weeks. Central testing of tumor tissue will determine CLDN18.2 and HER2 status (if unknown); patients will be considered CLDN18.2⁺ if ≥75% of tumor cells demonstrate moderate-to-strong membranous immunohistochemical staining. The primary objective is to compare progression-free survival between treatment arms. Secondary endpoints are overall survival; objective response rate; duration of response; and the safety/tolerability, pharmacokinetics, and immunogenicity of zolbetuximab. As of January 31, 2020, 127 sites were active and open to enrollment. Clinical trial information: NCT03653507.