Glioblastoma is incurable and in urgent need of improved therapeutics. We identify a small compound, Gliocidin, that kills glioblastoma cells while sparing nontumor replicative cells. Gliocidin activity targets a de novo purine synthesis vulnerability in glioblastoma through indirect inhibition of inosine monophosphate dehydrogenase 2 (IMPDH2). IMPDH2 blockade reduces intracellular guanine nucleotide levels causing nucleotide imbalance, replication stress, and tumor cell death. Gliocidin is a prodrug anabolized into its tumoricidal metabolite, Gliocidin-adenine dinucleotide (GAD), by the enzyme nicotinamide nucleotide adenylyltransferase (NMNAT1) of the NAD+ salvage pathway. The cryo-EM structure of GAD together with IMPDH2 demonstrates its entry, deformation, and blockade of the NAD+ pocket. In vivo, Gliocidin penetrates the blood brain barrier and extends orthotopic murine glioblastoma mouse survival. The DNA alkylating agent, temozolomide, induces Nmnat1 expression causing synergistic tumor killing and additional survival benefit in orthotopic patient-derived xenograft models. This study brings Gliocidin to light as a prodrug with potential to improve the survival of glioblastoma patients. At time of import, last updated (by provider) on: Contributors: ; [Mungunsarnai Ganbol, Yu-Jung Chen, Luis F Parada] (inherited)