Andecaliximab (ADX) is a monoclonal antibody that inhibits matrix metalloproteinase 9, an extracellular enzyme involved in matrix remodeling, tumor growth, and metastases. A phase I and Ib study of modified oxaliplatin, leucovorin, and fluorouracil (mFOLFOX6) with ADX revealed encouraging antitumor activity in patients with gastric or gastroesophageal junction (GEJ) adenocarcinoma.This phase III, randomized, double-blinded, placebo (PBO)-controlled multicenter study investigated the efficacy and safety of mFOLFOX6 with and without ADX in patients with untreated human epidermal growth factor receptor 2-negative gastric or GEJ adenocarcinoma. Random assignment was 1:1 to mFOLFOX6 + ADX or mFOLFOX6 + PBO. ADX/PBO 800 mg was infused on days 1 and 15 of each 28-day cycle. Protocol therapy was given until disease progression or intolerance. The primary end point was overall survival (OS), and secondary end points were progression-free survival (PFS), objective response rate (RECIST 1.1), and safety.Between September 2015 and May 2017, 432 patients were randomly assigned, 218 to ADX and 214 to PBO. The median OS was 12.5 versus 11.8 months in the ADX and PBO groups, respectively. The median PFS was 7.5 versus 7.1 months in the ADX and PBO groups, respectively. The objective response rate was 51% in the ADX group and 41% in the PBO group. Among the subgroup analyses, patients of age ≥ 65 years had an improved OS and PFS with ADX versus PBO; the P values and CIs were not adjusted for multiplicity. There were no meaningful differences in the safety profile of the ADX versus PBO groups.The addition of ADX to mFOLFOX6 did not improve OS in unselected patients with untreated human epidermal growth factor receptor 2-negative gastric or GEJ adenocarcinoma.