Background: This study evaluated the effect of time interval between surgery to radiotherapy initiation on overall survival in glioblastoma patients. Methods: From January 1998 to June 2016, there were 392 eligible patients. In this retrospective study, only histopathologically confirmed glioblastoma patients were included. All patients were planned for maximal surgical resection, 3D conformal or intensity modulated radiotherapy with 6 cycles of concurrent temozolomide followed by 6 cycles of adjuvant temozolomide. The surgery to radiotherapy interval was evaluated both as a categorical and continuous variable. For categorical variable, the patients were divided into 2 groups based on surgery to starting radiotherapy time interval of ≤ 10 and > 10 days. The association of time interval and other known factors affecting oncological outcomes were analyzed using cox proportional- hazards regression. The primary end point was overall survival. Results: The median time interval between surgery and starting radiotherapy was 15.5 days. The overall median survival in all patients was 14.7 months. The median survival was 19.7 months when surgery to starting radiotherapy time interval was ≤ 10 days and 13.4 months when the time interval was > 10 days (P < 0.001). In univariate analysis, the hazard ratio was 1.78 (95%confidence interval (CI) 1.38 – 2.29; P < 0.001) when compared to the referent group of time interval ≤ 10 days. When adjusted for known prognostic factors like age, performance status and extent for resection in a multivariate analysis, the corresponding hazard ratio was 1.88 (confidence interval, CI 1.45 – 2.43; P < 0.001). This translates to 1.8 times increased risk of death when surgery to radiotherapy time interval is more than 10 days. Surgery to radiotherapy time interval when tested as a continuous variable gave a hazard ratio of 1.015 ( confidence interval, CI 1.003 – 1.026; P = 0.012) translating to 1.5% increased risk of death for every 1 day delay in starting radiotherapy 10 days after surgery. Conclusions: Surgery to radiotherapy time interval of more than 10 days has a detrimental effect on overall survival in glioblastoma patients.