Glioblastoma (GBM) remains among the deadliest of human malignancies, and the emergence of the cancer stem cell (CSC) phenotype represents a major challenge to durable treatment response. Because the environmental and lifestyle factors that impact CSC populations are not clear, we sought to understand the consequences of diet on CSC enrichment. We evaluated disease progression in mice fed an obesity-inducing high-fat diet (HFD) versus a low-fat, control diet. HFD resulted in hyperaggressive disease accompanied by CSC enrichment and shortened survival. HFD drove intracerebral accumulation of saturated fats, which inhibited the production of the cysteine metabolite and gasotransmitter, hydrogen sulfide (H₂S). H₂S functions principally through protein S-sulfhydration and regulates multiple programs, including bioenergetics and metabolism. Inhibition of H₂S increased proliferation and chemotherapy resistance, whereas treatment with H₂S donors led to death of cultured GBM cells and stasis of GBM tumors in vivo. Syngeneic GBM models and GBM patient specimens present an overall reduction in protein S-sulfhydration, primarily associated with proteins regulating cellular metabolism. These findings provide clear evidence that diet-modifiable H₂S signaling serves to suppress GBM by restricting metabolic fitness, while its loss triggers CSC enrichment and disease acceleration. Interventions augmenting H₂S bioavailability concurrent with GBM standard of care may improve outcomes for patients with GBM.