PURPOSE In CheckMate 649, first-line nivolumab plus chemotherapy prolonged overall survival versus chemotherapy in patients with advanced/metastatic non–human epidermal growth factor receptor 2 (HER2)-positive gastric/gastroesophageal junction cancer (GC/GEJC) or esophageal adenocarcinoma (EAC). We present exploratory patient-reported outcomes (PROs). METHODS In patients (N = 1,581) concurrently randomly assigned 1:1 to nivolumab plus chemotherapy or chemotherapy and in those with tumor PD-L1 expression at a combined positive score (CPS) of ≥5, health-related quality of life (HRQoL) was assessed using the EQ-5D and Functional Assessment of Cancer Therapy-Gastric (FACT-Ga), which included the FACT-General (FACT-G) and Gastric Cancer subscale (GaCS). The FACT-G GP5 item assessed treatment-related symptom burden. Longitudinal changes in HRQoL were assessed using mixed models for repeated measures in the PRO analysis population (randomly assigned patients with baseline and ≥1 postbaseline assessments). Time to symptom or definitive deterioration analyses were also conducted. RESULTS In the PRO analysis population (n = 1,360), PRO questionnaire completion rates were mostly >80% during treatment. Patient-reported symptom burden was not increased with nivolumab plus chemotherapy versus chemotherapy. Mean improved changes from baseline were greater with nivolumab plus chemotherapy versus chemotherapy for FACT-Ga total, GaCS, and EQ-5D visual analog scale in patients with a CPS of ≥5; results were similar for the overall PRO analysis population. In CPS ≥5 and all randomly assigned populations, nivolumab plus chemotherapy reduced the risk of symptom deterioration versus chemotherapy, on the basis of FACT-Ga total score and GaCS; time to definitive deterioration was longer, and the risk of definitive deterioration in HRQoL was reduced with nivolumab plus chemotherapy across EQ-5D and most FACT-Ga measures (hazard ratio [95% CI] <1). CONCLUSION Compared with chemotherapy alone, first-line nivolumab plus chemotherapy showed stable or better on-treatment HRQoL in patients with advanced/metastatic non–HER2-positive GC/GEJC/EAC and also showed decreased risk of definitive HRQoL deterioration.