Glioblastoma multiforme (GBM) is characterized by poor therapeutic response and poor overall survival. It is crucial that more effective therapies be developed for the treatment of GBM. Inhibitor of DNA binding protein-1 (ID1) has been shown to maintain the self-renewal capacity of neural stem cells and might be involved in the therapeutic resistance of GBM. In the present study, we explored survival data from the The Cancer Genome Atalas database that were based on ID1 expression for patients diagnosed with primary GBMs. Interestingly, patients with high ID1 expression had better survival than patients with low ID1 expression, and a strong correlation was found between radiotherapy efficacy, ID1 expression, and overall survival. We further investigated the relationship between ID1 expression and the radiosensitivity of glioblastoma using glioblastoma cell lines. The clonogenic formation assay showed that U87 ID1-shRNA cells were much less sensitive to radiation. Moreover, both the results of the γH2AX foci staining assay and the comet assay further revealed that ID1 negatively regulates DNA repair processes by downregulating the expression of genes such as DNA ligase IV (LIG4) and ataxia-telangiectasia-mutated. Additionally, ID1 induces G2/M arrest in U87 cells. Taken together, these results suggest that ID1 may be a new prognostic marker for GBM and have important implications for the therapeutic strategies used to treat GBM patients.