Objective To investigate the effect of postoperative adjuvant radiotherapy on the prognosis of patients with advanced gastric signet ring cell carcinoma. Methods The data of 1 694 patients with advanced gastric signet ring cell carcinoma were collected from the Surveillance, Epidemiology, and End Results (SEER) database between 2004 and 2015. The patients were divided into 2 groups, with or without postoperative radiotherapy. Kaplan-Meier method was used to analyze the survival of patients in the 2 groups. Log-rank test and Cox regression were used for survival analysis and subgroup analysis. Results Kaplan-Meier survival curves showed that the prognosis of patients with postoperative radiotherapy was better than that without postoperative radiotherapy group (P<0.05). Log-rank univariate analysis showed that age, race, marital status, TNM stage, T stage, N stage, primary tumor location, maximum tumor diameter, whether to receive chemotherapy or not and whether to receive postoperative radiotherapy or not were the prognostic factors for advanced gastric signet ring cell carcinoma patients (all P<0.01). Multivariate Cox regression analysis showed that age, tumor T and N stages, maximum tumor diameter, whether to receive chemotherapy or not and whether to receive postoperative radiotherapy or not were independent prognostic factors (all P<0.01). Subgroup analysis showed except for N0 group (P>0.05), patients with age ≤ 70 years and >70 years, different T stages, lymph node metastasis (N1-N3), maximum tumor diameter ≤ 5 cm and >5 cm, receiving chemotherapy and those who did not receive chemotherapy, could benefit from postoperative adjuvant radiotherapy (all P<0.01). Conclusion For advanced gastric signet ring cell carcinoma patients who have received surgery, the prognosis of those with postoperative radiotherapy is better than that without postoperative radiotherapy.