Background This meta-analysis aimed to provide a pooled analysis of clinical studies correlating postoperative radiotherapy (PORT) with survival in patients with completely resected thymoma. Methods According to the recommendations of the Cochrane Collaboration, we established a rigorous study protocol. An electronic search was conducted using online databases. Hazard ratios (HRs) and 95% confidence intervals (CIs) were used in this meta-analysis and were calculated from published survival data. A meta-analysis was conducted to assess the impact of PORT in completely resected thymoma on overall survival (OS), disease-free survival (DFS). and disease-specific survival (DSS). We also performed a subgroup analysis for OS of patients with stage II and stage III thymoma. Results Fourteen studies, which included 3,823 patients (2,096 patients who received PORT and 1,727 patients who did not receive PORT), met the selection criteria. From the available data, the thymoma patients with PORT who did not undergo resection did not have significantly improved OS (HR 0.99; 95% CI 0.87 to 1.13; p  = 0.87), DFS (HR 1.21; 95% CI 0.97 to 1.51; p  = 0.09), or DSS (HR 0.66; 95% CI 0.39 to 1.13; p  = 0.13) compared with the patients who did not undergo PORT. However, our subgroup analysis showed a significant difference in OS in patients with stage II thymoma (HR 0.57; 95% CI 0.41 to 0.80; p  = 0.001) and patients with stage III thymoma (HR 0.73; 95% CI 0.59 to 0.90; p  = 0.004). Conclusion Our results showed that for completely resected thymoma, PORT had no advantage in the overall group of patients but increased OS in the patients with stage II and III thymoma after a complete resection. On the basis of this study, PORT is beneficial in patients with stage II and III patients after a complete resection.