Background: Limited data are available to guide management of patients with stage I–III gastric cancer not undergoing potentially curative surgical resection. We compared survival outcomes associated with chemotherapy alone versus chemoradiation (CRT) in the treatment of nonmetastatic gastric cancer. Methods: Patients with gastric adenocarcinoma from 2004 to 2015 were identified using the National Cancer Database. Patients were excluded if they had surgery, metastatic disease, or T0, Tis, or T1a disease. Logistic regression was used to evaluate predictors of CRT use. Cox proportional hazards modeling was performed to compare overall survival (OS) between chemotherapy alone and CRT in overall and propensity score–matched cohorts. Results: We identified 4,795 patients with stage I–III gastric adenocarcinoma who did not undergo surgery, at a median follow-up of 11.8 months. A total of 3,316 patients (69.2%) received chemotherapy alone and 1,479 patients (30.8%) received CRT. Predictors of increased CRT use were age ≥65 years (odds ratio [OR] 1.68; 95% CI, 1.43–1.99; P<.001), Charlson-Deyo comorbidity score ≥2 (OR, 1.46; 95% CI, 1.18–1.81), and treatment at a community facility (OR, 1.27; 95% CI, 1.07–1.51; P=.006). Patients receiving CRT had a 2-year OS rate of 28.3% compared with 21.5% among those receiving chemotherapy. Multivariate analysis showed that CRT was associated with improved OS (hazard ratio [HR], 0.82; 95% CI, 0.77–0.89; P<.001). After propensity score matching, a persistent survival benefit was observed (HR, 0.80; 95% CI, 0.74–0.88; P<.001). Conclusions: In patients with stage I–III gastric cancer not undergoing surgical resection, CRT was associated with improved survival compared with chemotherapy alone. However, only 30.8% of patients received CRT in this setting.