Background: The late-onset cerebellar ataxias (LOCAs) have until recently resisted molecular diagnosis. Contributing to this diagnostic gap is that non-coding structural variations, such as repeat expansions, are not fully accessible to standard short-read sequencing analysis. Methods: We combined bioinformatics analysis of whole-genome sequencing and long-read sequencing to search for repeat expansions in patients with LOCA. We enrolled 66 French-Canadian, 228 German, 20 Australian and 31 Indian patients. Pathogenic mechanisms were studied in post-mortem cerebellum and induced pluripotent stem cell (iPSC)-derived motor neurons from 2 patients. Results: We identified 128 patients who carried an autosomal dominant GAA repeat expansion in the first intron of the FGF14 gene. The expansion was present in 61%, 18%, 15% and 10% of patients in the French-Canadian, German, Australian and Indian cohorts, respectively. The pathogenic threshold was determined to be (GAA)≥250, although incomplete penetrance was observed in the (GAA)250-300 range. Patients developed a slowly progressive cerebellar syndrome at an average age of 59 years. Patient-derived post-mortem cerebellum and induced motor neurons both showed reduction in FGF14 RNA and protein expression compared to controls. Conclusions: This intronic, dominantly inherited GAA repeat expansion in FGF14 represents one of the most common genetic causes of LOCA uncovered to date.