Chemotherapy with Temozolomide (TMZ) is a mainstay for the treatment of primary brain tumors (glioblastomas; GBM). However, drug efficacy is reduced through a profound ability of GBM cells for DNA-repair and by the blood tumor barrier (BTB), which restricts intratumoral TMZ accumulation. Coherently, GBM almost invariably relapse with fatal outcome. We here show that GBM associated myeloid cells (GAM) simultaneously induce chemoresistance on the genetic and the vascular level by activating GP130 receptor signaling, which can be addressed therapeutically. After performing transcriptomic screens with human GAMs and microglia as well as immunohistochemical inspection of human biopsies, we observed an upregulation of the mitochondrial encoded signaling peptide Humanin in GBM. Pharmacological experiments with a humanized organotypic GBM model demonstrated that the interaction of GAM with GBM cells leads to Humanin expression. Nanomolar concentrations of Humanin promoted TMZ resistance of human stem-like GBM cells via GP130 and MAPK (ERK) activation. This was confirmed through in vitro and in vivo studies, indicating that the Humanin-GP130-ERK signaling axis drives ATR dependent DNA repair. GBM mouse models recapitulating intratumoral Humanin-release exhibited vascular alterations including an increased pericyte coverage as compared to controls. High pericyte numbers were associated with poor intratumoral accumulation of blood-borne tracers, whereas GP130 blockade attenuated BTB formation and supported the outcome of GBM chemotherapy. Altogether, we describe an overarching mechanism for TMZ resistance and outline a translatable strategy with predictive markers to improve chemotherapy for GBM.