Background: Systemic lymphopenia, which is independently associated with poor overall survival, is observed in approximately 30-50% of patients with glioblastoma (GBM) after chemoradiotherapy. We recently found that cranial irradiation of GBM tumors increases circulating myeloid-derived suppressor cells (MDSC) contributing to systemic lymphopenia. Additionally, we found that cranial radiation increases circulating levels of the RAGE ligands S100A8/9 and HMGB1, which have been reported to increase MDSC and their immunosuppressive effects through binding RAGE. We, therefore, hypothesized that targeting RAGE receptors on MDSC could prevent immunosuppression and improve survival in preclinical GBM models. Methods: We orthotopically implanted GBM cells (GL261 and CT2A) into immunocompetent C57BL6 mice. The GBM tumor-bearing mice were treated with five doses of fractionated radiation therapy (RT, 2 Gy/day for 5 consecutive days) to the whole head in combination with RAGE inhibitor azeliragon (0.1mg/mice/day i.p starting one day before RT for up to 10 days). We evaluated the immune cells in the tumor microenvironment (TME) and spleen at 14 days post-tumor implantation. We also evaluated the circulating immune cells and RAGE ligands (HMGB1 and S100A8/A9) in the plasma at 7, 14, and 21 days post-tumor implantation and monitored the mice for survival up to 60 days. Results: GBM tumor-bearing mice (GL261 & CT2A) treated with RT plus azeliragon had significantly improved survival when compared to RT alone or azeliragon alone. RT plus azeliragon significantly reduced the accumulation of granulocytic-MDSC (G-MDSC), monocytic-MDSC (M-MDSC), tumor-associated macrophages (TAM), and regulatory T cells (T_reg), while increasing infiltrating CD8 T cells in the TME as compared to RT alone. RT alone significantly increased the accumulation of G-MDSC and M-MDSC in the spleen, which was significantly reduced with concurrent azeliragon. Concurrent administration of azeliragon with RT did not significantly affect the radiation-induced increase of circulating M-MDSC as well as plasma HMGB1 and S100A8/A9 concentration, though there was a modest reduction of circulating G-MDSC. Conclusions: Our study indicates that azeliragon, by blocking RAGE interaction with its ligands, helps to overcome the MDSC-mediated immunosuppression that occurs after RT. The combination of azeliragon with RT enhances survival in preclinical models. Clinical trials evaluating the safety and efficacy of azeliragon in combination with RT for newly diagnosed GBM are currently under development.