Adjuvant concurrent chemoradiotherapy (CCRT) is the standard care for patients with resected advanced gastric cancer, but its survival benefits remain undetermined in patients undergoing D2 lymph node dissection (D2 dissection). We evaluated safety and efficacy of adjuvant CCRT with 5-fluorouracil (5-FU) versus chemotherapy alone in 110 gastric cancer patients with D2 dissection treated in Taiwan between January 2009 and January 2013. All the 71 patients receiving adjuvant CCRT were treated with daily infusional 5-FU and radiotherapy. Adjuvant CCRT was associated with higher risks of major hematologic (56.3% vs. 23.8%, p = 0.002) and gastrointestinal (46.9% vs. 14.3%, p = 0.027) toxicities and death (12.5% vs. 0.0%, p = 0.041) in patients above 70 years old, but this was not the case in those ≤70 years of age. Univariate Cox proportional regressions identified adjuvant CCRT as a factor for better overall survival (OS) (hazard ratio [HR]=0.52; 95% confidence interval [CI]: 0.27–0.99) and disease-free survival (DFS) (HR=0.46, 95% CI: 0.24–0.88), but it was not a significant factor for OS or DFS after adjusting for other factors in the multivariate analysis. However, in stratified analyses by age, we found adjuvant CCRT was an independent prognostic factor for better OS (HR=0.07; 95% CI: 0.01–0.38) in patients ≤70 years old, but not in those above 70 years of age. Therefore, it was concluded that age may to be a modifier of the effects of adjuvant CCRT.