Glioblastoma multiforme is the most common and aggressive malignant brain tumor in adults. One of the factors contributing to the progression is the complex tumor microenvironment. Specially, the coexistence of two cell types, tumoral cells and resident astrocytes, sharing genetic and phenotypical backgrounds. This aspect hinders the phenotypical and functional distinction of both cell types during tumor development. Here, we found a differential expression of the astrocytic markers GFAP and S100β that allowed us to histologically distinguish between tumoral cells and resident astrocytes in GBM human samples and in a GBM mouse model in vivo. Interestingly, the core of the tumor showed a high expression of S100β and a low expression of GFAP, corresponding to the tumoral cells. In contrast, the periphery was rich in GFAP high and S100β low reactive astrocytes that corralled the tumor. Based on that heterogeneity, we evaluated the effects of the JAK/STAT3 pathway inhibition, a master regulator of astrocyte reactivity, using adeno-associated virus encoding SOCS3 in the GBM experimental mouse model. SOCS3 overexpression decreased the percentage of GBM cells expressing S100β in the core of the tumor. Moreover, it induced changes in the peripheral corralling mediated by reactive astrocytes, decreasing the number of GFAP+ cells, but increasing the amount and length of their projections. Overall, our data presume that STAT3-induced reactive astrocytes, encircling the tumor mass, may play a detrimental role in GBM progression and malignancy. Therefore, targeting astrocyte reactivity could be a potential approach for GBM therapy.