Introduction: Radiotherapy (RT) plays an indispensable role in postoperative breast cancer treatment. This study aimed to assess the feasibility of preoperative RT for stage III breast cancer by comparing preoperative RT with postoperative RT in terms of overall survival (OS). Methods: Based on the information in the Surveillance, Epidemiology, and End Results database from 2000 to 2018, patients with stage III breast cancer who had undergone radical surgery and RT were divided into two groups: a preoperative RT group and a postoperative RT group. OS was calculated using Kaplan-Meier analysis. The Cox proportional hazards model was used to evaluate independent factors associated with OS. Propensity score matching (PSM) was used to balance stratification factors. Results: In total, 9,605 patients were enrolled, of whom 9,456 received postoperative RT and 149 received preoperative RT. After a median follow-up of 72 months, postoperative RT was found to be superior to preoperative RT in terms of OS (p < 0.000). Compared to the postoperative RT group, the preoperative RT group showed a significantly higher risk of overall mortality without PSM in univariate (OS: hazard ratio [HR] = 1.653, 95% confidence interval [CI]: 1.288–2.123, p < 0.000) and multivariate analyses (OS: HR = 1.409, 95% CI: 1.096–1.810, p = 0.007). After PSM, the OS of the postoperative RT group was superior to the OS in the preoperative RT group (p = 0.041). Compared with the postoperative RT group, the preoperative RT group showed a significantly higher risk of overall mortality without PSM in univariate (HR = 1.312, 95% CI: 1.010–1.704, p = 0.042) and multivariate analyses (HR = 1.466, 95% CI: 1.127–1.906, p = 0.004). Conclusion: Preoperative RT does not improve OS in patients with stage III breast cancer and has a worse prognosis. Preoperative RT has not changed the existing treatment paradigm in the current therapeutic context for patients with stage III breast cancer.