Glioblastoma (GBM) is a lethal brain tumor containing a subpopulation of glioma stem cells (GSCs). Pan-cancer analyses have revealed that stemness of cancer cells correlates positively with immunosuppressive pathways in many solid tumors, including GBM, prompting us to conduct a gain-of-function screen of epigenetic regulators that may influence GSC self-renewal and tumor immunity. The circadian regulator CLOCK emerged as a top hit in enhancing stem cell self-renewal, which was amplified in about 5% of human GBM cases. CLOCK and its heterodimeric partner BMAL1 enhanced GSC self-renewal and triggered pro-tumor immunity via transcriptional up-regulation of OLFML3, a novel chemokine recruiting immune-suppressive microglia into the tumor microenvironment. In GBM models, CLOCK or OLFML3 depletion reduced intratumoral microglia density and extended overall survival. We conclude that the CLOCK:BMAL1 complex contributes to key GBM hallmarks of GSC maintenance and immunosuppression and, together with its downstream target OLFML3, represents new therapeutic targets for this disease.