Huntington disease is an uncommon autosomal dominant neurodegenerative disorder caused by expanded polyglutamine repeats in the huntingtin protein. The proximate mechanisms responsible for neurodegeneration are unknown. Copper ions may play a role in Huntington disease by promoting oligomerization of expanded polyglutamine repeat protein fragments. Ammonium tetrathiomolybdate is a copper complexing agent with demonstrated tolerability and efficacy in another neurodegenerative disorder, Wilson disease. We evaluated ammonium tetrathiomolybdate in the R6/2 transgenic mouse model of Huntington disease. Ammonium tetrathiomolybdate treatment delayed the onset of motor dysfunction in R6/2 mice. There was a trend towards reduced striatal degeneration, suggesting a neuroprotective effect of ammonium tetrathiomolybdate in this model. Given its known tolerability in humans with neurodegeneration, ammonium tetrathiomolybdate could be considered as a candidate for clinical trials in Huntington disease.