Glioblastoma multiforme (GBM) is a devastating disease with high mortality and poor prognosis. Cancer stem cells (CSCs) have recently been defined as a fraction of tumor cells highly resistant to therapy and subsequently considered to be responsible for tumor recurrence. These cells have been characterized in GBM and suggested to reside in and be supported by the tumor microvascular niche. Here we evaluated the response of tumor microvascular endothelial cells (tMVECs) to radio- and chemotherapy, and analyzed how this affects their interaction with CSCs. Our data demonstrate that tMVECs exhibit extreme resistance to both therapies, with the main response to irradiation being senescence. Importantly, senescent tMVECs can be detected in human GBM samples as well as in mice upon irradiation. Even though permanently arrested, they are still viable and able to support CSC growth with the same efficacy as non-senescent tMVECs. Intriguingly, GBM CSCs themselves are capable of differentiating into cells with similar features as tMVECs that subsequently undergo senescence when exposed to radiation. This indicates that endothelial-like cells are therapy resistant and, more importantly, support expansion of GBM cells.