Glioblastoma multiforme (GBM) is a highly aggressive form of primary brain tumor in adults, which unfortunately has an abysmal prognosis and poor survival rates. The reason behind the poor success rate of several FDA-approved drug is mainly attributed to insufficient drug distribution to the tumor site across the blood–brain barrier (BBB) and induction of resistance. In this study, we have developed a novel nanotherapeutic approach to achieve our goal. PLGA-based nanoencapsulation of both Temozolomide (TMZ) and EGFR inhibitor 3,3′-diindoyl methane (DIM) in a combinatorial approach enhances the delivery of them together. Their synergistic mode of actions, significantly enhances the cytotoxic effect of TMZ in vitro and in vivo. Moreover, the dual-loaded nanoformulation works more efficiently on DNA damage and apoptosis, resulting in a several-fold reduction in tumor burden in vivo, systemic drug toxicity, and increased survival. These findings suggest the preclinical potential of this new treatment strategy.