Dentatorubral-pallidoluysian atrophy (DRPLA) is a fatal neurodegenerative disease arising from a CAG repeat expansion in the atrophin-1 (ATN1) gene. Because DRPLA, like many repeat expansion disorders (REDs), arises predominantly from toxic gain-of-function mechanisms, we hypothesized that ATN1 knockdown would have therapeutic potential. To test this, we established the first fully humanized mouse model of a RED, in which one allele of mouse Atn1 is completely replaced by human ATN1, including 112 pure CAG repeats. This novel approach to exploring RED biology provides significant advantages, notably the ability to test sequence-specific therapeutics targeting human sequences, even in introns and untranslated regions of pre-mRNA. We found that our model—the Atn1^Q112/+ mouse—recapitulates key features of human DRPLA, including behavioral alterations, reduced brain size, and aggregate accumulation. We treated Atn1^Q112/+ mice with antisense oligonucleotides (ASOs) targeting mouse Atn1 (to probe for loss of function concerns), human ATN1, or a combination. Treatment with human, but not mouse, ATN1-targeting ASOs provides remarkable protection from a range of disease-related behavioral phenotypes and marked rescue of transcriptional dysregulation in the cerebellum. These results have helped motivate an ongoing human clinical study of ASOs targeting ATN1 for DRPLA.