Proneural (PN) to mesenchymal (MES) transition (PMT) is a crucial phenotypic shift in glioblastoma stem cells (GSCs). However, the mechanisms driving this process remain poorly understood. Here, we report that Fos-like antigen 1 (FOSL1), a component of AP1 transcription factor complexes, is a key player in regulating PMT. FOSL1 is predominantly expressed in the MES subtype, but not PN subtype, of GSCs. Knocking down FOSL1 expression in MES GSCs leads to the loss of MES features and tumor-initiating ability, whereas ectopic expression of FOSL1 in PN GSCs is able to induce PMT and maintain MES features. Moreover, FOSL1 facilitates ionizing radiation (IR)-induced PMT and radioresistance of PN GSCs. Inhibition of FOSL1 enhances the anti-tumor effects of IR by preventing IR-induced PMT. Mechanistically, we find that FOSL1 promotes UBC9-dependent CYLD SUMOylation, thereby inducing K63-linked polyubiquitination of major nuclear factor κB (NF-κB) intermediaries and subsequent NF-κB activation, which results in PMT induction in GSCs. Our study underscores the importance of FOSL1 in the regulation of PMT and suggests that therapeutic targeting of FOSL1 holds promise to attenuate molecular subtype switching in patients with glioblastomas.