Trinucleotide repeat disorders comprise ~20 severe, inherited, human neuromuscular and neurodegenerative disorders, which result from an abnormal expansion of repetitive sequences in the DNA. The most common of these, Huntington’s disease (HD), results from expansion of the CAG repeat region in exon 1 of theHTTgene via an unknown mechanism. Since non-coding RNAs have been implicated in the initiation and progression of many diseases, herein we focused on a circular RNA (circRNA) molecule arising from non-canonical splicing (backsplicing) ofHTTpre-mRNA. The most abundant circRNA fromHTT,circHTT(2-6), was found to be more highly expressed in the frontal cortex of HD patients, compared with healthy controls, and positively correlated with CAG repeat tract length. Furthermore, the mouse orthologue (mmu_circHTT(2-6)) was found to be enriched within the brain and specifically the striatum, a region enriched for medium spiny neurons that are preferentially lost in HD. Transgenic overexpression ofcircHTT(2-6)in two human cell lines—SH-SY5Y and HEK293—reduced cell proliferation and nuclear size without affecting cell cycle progression or cellular size, or altering the CAG repeat region length withinHTT.CircHTT(2-6)overexpression did not alter total HTT protein levels, but reduced its nuclear localisation. As these phenotypic and genotypic changes resemble those observed in HD patients, our results suggest thatcircHTT(2-6)may play a functional role in the pathophysiology of this disease.