The expansion of a hexanucleotide repeat GGGGCC (G4C2) in the C9orf72 gene is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The G4C2 expansion leads to repeat-associated non-AUG (RAN) translation and the production of toxic dipeptide repeat (DPR) proteins, but the mechanisms of RAN translation remain enigmatic. Here, we report that the RNA helicase DHX36 is a robust positive regulator of C9orf72 RAN translation. DHX36 has a high affinity for the G4C2 repeat RNA, preferentially binds to the repeat RNA's G-quadruplex conformation, and efficiently unwinds the G4C2 G-quadruplex structures. Native DHX36 interacts with the G4C2 repeat RNA and is essential for effective RAN translation in the cell. In induced pluripotent stem cells and differentiated motor neurons derived from C9orf72-linked ALS patients, reducing DHX36 significantly decreased the levels of endogenous DPR proteins. DHX36 is also aberrantly upregulated in tissues of C9orf72-linked ALS patients. These results indicate that DHX36 facilitates C9orf72 RAN translation by resolving repeat RNA G-quadruplex structures and may be a potential target for therapeutic intervention.