Glioblastoma is the most malignant and lethal subtype brain tumors with high risk of recurrence and therapeutic resistance. Emerging evidence has indicated that glycogen synthesis kinase 3 (GSK3) plays oncogenic roles in multiple tumor types; however, the underlying mechanisms remain largely unknown. It has also been demonstrated that p53 binding protein 1 (53BP1) plays a central role in DNA double-strand break (DSB) repair. This study aimed to reveal the significance of GSK3 trans-location from the cytoplasm to the nucleus, and to determine whether GSK3 induces DNA DSB repair in the nuclei of glioblastoma cells via phospho-53BP1. By performing in vitro experiments, we found that GSK3 translocated from the cytoplasm to the nucleus, and it then bound to 53BP1 following exposure to IR (IR). In addition, 53BP1-mediated DNA DSB repair was observed to be abrogated by the inhibition of GSK3. Further experiments on the phosphorylation site of 53BP1 by GSK3 revealed that the S/T-Q motif may play a critical role. Importantly, our in vivo and in vitro data clearly indicated that GSK3 induced the phosphorylation of 53BP1 at the Ser166 site. Moreover, brain tumor xenograft models revealed that following exposure to IR plus SB216763, a specific GSK3 inhibitor, tumor growth was markedly inhibited and the survival of mice markedly increased. Based on these results, we concluded that the phosphorylation of 53BP1 by GSK3 was indispensable for DNA DSB repair. Our study also suggested that the inhibition of GSK3 by SB216763 significantly inhibited the proliferation and induced the apoptosis of glioblastoma cells. Taken together, our data indicate that GSK3, a key phosphorylation protein for 53BP1, may be a potential target for enhancing the sensitivity of glioblastoma cells to radiation.