Background The impact of adjuvant chemotherapy and chemo-radiation therapy in the treatment of resectable gastric cancer remains varied. We sought to define the clinical impact of lymph node ratio on the relative benefit of adjuvant chemotherapy or chemo-radiation therapy among patients having undergone curative-intent resection for gastric cancer. Methods Using the multi-institutional US Gastric Cancer Collaborative database, 719 patients with gastric adenocarcinoma who underwent curative-intent resection between 2000 and 2013 were identified. Patients with metastasis or an R2 margin were excluded. The impact of lymph node ratio on overall survival among patients who received chemotherapy or chemo-radiation therapy was evaluated. Results Median patient age was 65 years, and the majority of patients were male (56.2%). The majority of patients underwent either subtotal (40.6%) or total gastrectomy (41.0%), with the remainder undergoing distal gastrectomy or wedge resection (18.4%). On pathology, median tumor size was 4 cm; most patients had a T3 (33.0%) or T4 (27.9%) lesion with lymph node metastasis (59.7%). Margin status was R0 in 92.5% of patients. A total of 325 (45.2%) patients underwent resection alone, 253 (35.2%) patients received 5-FU or capecitabine-based chemo-radiation therapy, whereas the remaining 141 (19.6%) received chemotherapy. Median overall survival was 40.9 months, and 5-year overall survival was 40.3%. According to lymph node ratio categories, 5-year overall survival for patients with a lymph node ratio of 0, 0.01–0.10, >0.10–0.25, >0.25 were 54.1%, 53.1 %, 49.1 % and 19.8 %, respectively. Factors associated with worse overall survival included involvement of the gastroesophageal junction (hazard ratio 1.8), T-stage (3–4: hazard ratio 2.1), lymphovascular invasion (hazard ratio 1.4), and lymph node ratio (>0.25: hazard ratio 2.3; all P <.05). In contrast, receipt of adjuvant chemo-radiation therapy was associated with an improved overall survival in the multivariable model (versus resection alone: hazard ratio 0.40; versus chemotherapy: hazard ratio 0.45, both P <.001). The benefit of chemo-radiation therapy for resected gastric cancer was noted only among patients with lymph node ratio >0.25 (versus resection alone: hazard ratio R 0.34; versus chemotherapy: hazard ratio 0.45, both P <.001). In contrast, there was no noted overall survival benefit of chemotherapy or chemo-radiation therapy among patients with lymph node ratio ≤0.25 (all P >.05). Conclusion Adjuvant chemotherapy or chemo-radiation therapy was utilized in more than one-half of patients undergoing curative-intent resection for gastric cancer. Lymph node ratio may be a useful tool to select patients for adjuvant chemo-radiation therapy, because the benefit of chemo-radiation therapy was isolated to patients with greater degrees of lymphatic spread (ie, lymph node ratio >0.25).