Glioblastoma (GBM), the most malignant primary brain tumor, is characterized by widespread heterogeneity, leading to poor and unpredictable clinical outcomes. Recent studies have provided evidences that the tumor microenvironment has a critical role in regulating tumor growth by establishing a complex network of interactions with tumor cells. Here we investigate how GBM cells modulate glial cells, and how this modulation can influence back on the malignant phenotype of GBM cells. Primary mouse glial cultures were established and cultured in serum-free media (unprimed) or exposed to secretome derived from GL261 GBM cells (primed). Conditioned media (CM) from each glial culture were collected and a proteomic analysis was conducted. Glial cells CM (unprimed and primed) were also used in GL261 GBM cells to evaluate its impact in critical hallmarks of GBM cells, including viability, migration, and activation of tumor-related intracellular signaling pathways. The proteomic analysis revealed that the pre-exposure of glial cells to CM from GBM cells led to the upregulation of several proteins related to inflammatory response, cell adhesion and extracellular structure organization within the secretome of primed glial cells, consistent with a pattern of reactive astrogliosis. At the functional levels, CM derived from unprimed glial cells favored an increase in cell migration capacity, while CM from primed glial cells was more efficient in promoting GBM cells viability. These effects on GBM cells were accompanied by activation of particular intracellular cancer-related pathways, mainly the MAPK/ERK pathway, which is a known regulator of cell proliferation. Together, our results demonstrate that glial cells can have a different impact on the progression of GBM tumors, suggesting that the secretome of GBM cells is able to modulate the secretome of neighboring glial cells, in a way that regulates the “go-or-grow” phenotypic switch of GBM cells. Together, our results suggest that glial cells can impact on the pathophysiology of GBM tumors, and that the secretome of GBM cells is able to modulate the secretome of neighboring glial cells, in a way that regulates the “go-or-grow” phenotypic switch of GBM cells.