Huntington's disease (HD) is a fatal neurodegenerative disease caused by a genetic expansion of the CAG repeat region in the huntingtin (HTT) gene. Studies in HD mouse models have shown that artificial miRNAs can reduce mutant HTT but evidence for their effectiveness and safety in larger animals is lacking. HD transgenic sheep express the full-length human HTT with 73 CAG repeats. We used AAV9 to unilaterally deliver to HD sheep striatum an artificial miRNA targeting exon 48 of the human HTT mRNA under control of two alternative promoters- U6 or CbetaA. The treatment reduced human mutant (m) HTT mRNA and protein 50-80% in the striatum at one and six-months post-injection. Silencing was detectable in both caudate and putamen. Levels of endogenous sheep HTT protein were not affected. There was no significant loss of neurons labeled by DARPP32 or NeuN at six months after treatment, Iba1-positive microglia were detected at control levels. We conclude that safe and effective silencing of human mHTT protein can be achieved and sustained in a large animal brain by direct delivery of an AAV carrying an artificial miRNA.