Enhanced lipid metabolism, which involves the active import, storage, and utilization of fatty acids from the tumor microenvironment, plays a contributory role in malignant glioma transformation, thereby serving as an important gain of function. In this work, through studies initially designed to understand and reconcile possible mechanisms underlying the antitumor activity of a high-fat ketogenic diet, we discovered that this phenotype of enhanced lipid metabolism observed in glioblastoma may also serve as a metabolic vulnerability to diet modification. Specifically, exogenous polyunsaturated fatty acids (PUFAs) demonstrate the unique ability of short-circuiting lipid homeostasis in glioblastoma cells. This leads to lipolysis-mediated lipid droplet breakdown, an accumulation of intracellular free fatty acids, and lipid peroxidation–mediated cytotoxicity, which was potentiated when combined with radiation therapy. Leveraging these data, we formulated a PUFA-rich modified diet that does not require carbohydrate restriction, which would likely improve long-term adherence when compared with a ketogenic diet. The modified PUFA-rich diet demonstrated both antitumor activity and potent synergy when combined with radiation therapy in mouse glioblastoma models. Collectively, this work offers both a mechanistic understanding and a potentially translatable approach of targeting this metabolic phenotype in glioblastoma through diet modification and/or nutritional supplementation that may be readily integrated into clinical practice.