Background: Glioblastoma (GBM) is the most aggressive and most common primary central nervous system cancer in adults. Blocking the interaction between Programmed Cell Death Protein-1 (PD-1) and its ligand (PD-L1) has shown remarkable success in the treatment of several cancers. However, many challenges remain in improving the efficacy of using monoclonal antibodies (Ab) against the receptor PD-1 in GBM, mainly due to the “non-immunogenic” tumor characteristics of GBM. PD-L1 has been found to be overexpressed on the surface of human GBM tumor cells and tumor-associated macrophages (TAM). Radiotherapy (RT), as one of the standard therapy of GBM, could alter the tumor microenvironment and promote an antitumor immune response. We hypothesize that anti-PD-L1 therapy can enhance the RT effects and improve the outcome of treatment when combined. Methods: Using a preclinical orthotropic syngeneic CT-2A mouse GBM tumor model, we studied the efficacy of combined therapy with anti-PD-L1 and RT. Mice were stratified into four treatment groups: control, RT, anti-PD-L1 Ab, and anti-PD-L1 Ab plus RT. RT(8 Gy) was given one time simultaneously with the first dose of anti-PD-L1, followed by systemic anti-PD-L1 maintenance treatment to the mice. Overall survival and tumor growth were monitored. Immunohistochemistry on resected tumors during treatment was performed to characterize the immune response. Single-cell RNA sequencing (scRNA-seq) was also performed to further study the immunologic parameters in the mouse brain. Results: Our results showed that anti-PD-L1 Ab in combination with RT provided a remarkable antitumor immune response and improved overall survival, with 25.5, 34, and 30 days of median survival in control (no-treatment), RT, and anti-PD-L1 groups, respectively, and achieving long-term survival and complete tumor response in 80% of the mice in the anti-PD-L1+RT treatment group (median survival not reached) in GBM tumor-bearing mice. The combined therapy promoted the recruitment of tumor-infiltrating immune cells, reversed the hostile tumor immune environment with a higher M1/TAM ratio, CD8+ /CD4+ T cell ratio, and CD8+ T cell /Treg cell ratio in the tumor area comparing with those parameters in single modality treatment groups. Furthermore, scRNA-seq data demonstrated that anti-PD-L1 combined with RT resulted in robust higher CD8 effector T cells, while lower CD4 and CD8 exhausted T cells in the tumor region compared to other groups. Increased CD4 central memory T cells and CD8 central memory T cells were seen only in tumors treated with anti-PD-L1+RT providing immunologic explanations on the durable control of GBM achieved only by the combined therapy. Conclusions: The anti-PD-L1 therapy synergizes with RT by reversing the hostile tumor immune environment resulting in improved tumor control and long-term survival in the syngeneic mouse GBM model.