Importance While FOXC1 single-nucleotide variants and deletions are well-established causes of Axenfeld-Rieger syndrome, few FOXC1 duplications have been reported. This study investigated families with duplications encompassing the FOXC1 gene to refine the associated phenotypic spectrum and contribution to glaucoma. Objective To investigate the prevalence and phenotype of FOXC1 duplications in 2 large glaucoma registries. Design, Setting, and Participants This retrospective observational genetic cohort study included participants recruited from the Australian &amp; New Zealand Registry of Advanced Glaucoma (ANZRAG) and the Massachusetts Eye and Ear (MEE) cohort from 2008 through 2025. Participants with glaucoma, and available relatives, underwent genomic testing to identify duplications encompassing FOXC1 using exome sequencing and genotyping arrays (ANZRAG) or whole-genome sequencing (MEE). Data analyses were conducted from 2022 through 2025. Main Outcomes and Measures Prevalence of FOXC1 duplications, age at glaucoma onset, and phenotype, including ocular and systemic features. Results Twenty individuals from 10 families (50% female and 50% male; 70% self-described as broadly European [Australian/British, British, English/German, English/Polish, European, or Scottish], 25% as Asian [Chinese or Filipino], and 5% as Latin American [Salvadoran]) were identified with FOXC1 duplications. All genetically tested individuals were diagnosed with glaucoma, demonstrating high penetrance. Seventeen individuals were referred with juvenile open-angle glaucoma (JOAG), 1 with primary open-angle glaucoma, 1 with primary congenital glaucoma, and 1 with anterior segment dysgenesis. The diagnosis of 4 individuals from 1 family with ectropion uveae was revised to anterior segment dysgenesis. Systemic features were reported for 2 participants (10.5%), including subtle dental findings and mild facial dysmorphism. Duplications encompassing FOXC1 were among the most common monogenic contributors to JOAG. In the ANZRAG group, they accounted for 13.5% (95% CI, 6.7%-25.3%) of JOAG probands with a genetic diagnosis, second to MYOC (53.8%; 95% CI, 40.5%-66.7%). In the MEE group, FOXC1 duplications accounted for 9.5% (95% CI, 2.7%-28.9%) of JOAG probands with a genetic diagnosis. Conclusions and Relevance These findings suggest FOXC1 duplications are an underrecognized, highly penetrant, but variably expressive, genetic variation associated with JOAG. Findings for the relatively modest number of individuals in the retrospective study were associated with wide confidence intervals. This limitation is often inherent to studies of JOAG, a rare condition for which individual genetic variants account for only a subset of cases. Despite this, the findings highlight the genetic heterogeneity of JOAG and support the potential importance of considering routine genetic copy-number variant analysis for individuals with JOAG.