Introduction: People with Down's syndrome (DS) have a high prevalence of early-onset Alzheimer's disease. Early markers of Alzheimer's disease pathology identifiable before clinical change are needed for the evaluation of preventative treatments. The retina, an extension of the brain, may provide a noninvasive imaging site. Methods: Forty-nine adults with DS and 36 age-matched controls completed retinal nerve fibre layer (RNFL) assessments using optical coherence tomography. RNFL thickness was analyzed in relation to cognitive status and age and previously acquired cortical thickness and cerebral amyloid β binding data in a subgroup. Results: RNFL thickness was greater in the DS group and did not show age-related thinning. RNFL correlated positively with cognitive scores and cortical thickness and was reduced in participants with positive cerebral amyloid β binding. Discussion: Increased RNFL in adults with DS may represent early Alzheimer's disease-related changes. Thinning was present in those with cerebral amyloid β binding, independent of age.