Neuronal expression of familial Alzheimer's disease–mutant human amyloid precursor protein (hAPP) and hAPP-derived amyloid-β (Aβ) peptides causes synaptic dysfunction, inflammation and abnormal cerebrovascular tone in transgenic mice. Fatty acids may be involved in these processes, but their contribution to Alzheimer's disease pathogenesis is uncertain. We used a lipidomics approach to generate a broad profile of fatty acids in brain tissues of hAPP-expressing mice and found an increase in arachidonic acid and its metabolites, suggesting increased activity of the group IV isoform of phospholipase A₂ (GIVA-PLA₂). The levels of activated GIVA-PLA₂ in the hippocampus were increased in individuals with Alzheimer's disease and in hAPP mice. Aβ caused a dose-dependent increase in GIVA-PLA₂ phosphorylation in neuronal cultures. Inhibition of GIVA-PLA₂ diminished Aβ-induced neurotoxicity. Genetic ablation or reduction of GIVA-PLA₂ protected hAPP mice against Aβ-dependent deficits in learning and memory, behavioral alterations and premature mortality. Inhibition of GIVA-PLA₂ may be beneficial in the treatment and prevention of Alzheimer's disease.