Background: The aim of this study was to compare the efficacy of postoperative chemoradiotherapy (CRT) and chemotherapy (ChT) on survival of non-metastatic gastric cancer (GC) patients who had undergone an R1 resection. Methods: We retrospectively compared the survival of patients after an R1 resection of GC who only received postoperative chemotherapy (ChT group) with the survival of resected patients who had been treated with postoperative CRT (CRT group) at our center. Patients who had a resection between 2004 and 2016 were included. CRT consisted of radiotherapy (45Gy) combined with concurrent 5-fluorouracil-based chemotherapy. The impact of CRT treatment on overall survival (OS) was evaluated using multivariable Cox regression and stratified by propensity score. Results: A total of 116 GC patients who had undergone an R1 resection were studied (ChT, N = 83; CRT, N = 33). There were no significant differences in baseline characteristics and pathological T- and N-classification between the two groups. The median overall survival was significantly better in the CRT group (28 vs. 13 months; p = 0.007). In a multivariable analysis, CRT was an independent prognostic factor for improved OS (hazard ratio 0.67; 95 % confidence interval 0.43–0.94). This effect of CRT was further supported by propensity score analysis. Conclusions: Postoperative CRT was associated with improved OS in patients who had undergone an R1 resection for GC.