Accumulation of amyloid-β peptide (Aβ) in the brain is closely associated with cognitive decline in Alzheimer's disease (AD). Stereotaxic infusion of neprilysin-encoding viral vectors into the hippocampus has been shown to decrease Aβ in AD-model mice, but more efficient and global delivery is necessary to treat the broadly distributed burden in AD. Here we developed an adeno-associated virus (AAV) vector capable of providing neuronal gene expression throughout the brains after peripheral administration. A single intracardiac administration of the vector carrying neprilysin gene in AD-model mice elevated neprilysin activity broadly in the brain, and reduced Aβ oligomers, with concurrent alleviation of abnormal learning and memory function and improvement of amyloid burden. The exogenous neprilysin was localized mainly in endosomes, thereby effectively excluding Aβ oligomers from the brain. AAV vector-mediated gene transfer may provide a therapeutic strategy for neurodegenerative diseases, where global transduction of a therapeutic gene into the brain is necessary.