Objectives Early diagnosis in Alzheimer's disease (AD) is crucial in order to implement new therapeutic strategies. The retina is embryologically related to the brain. Thus, the possible usefulness of optical coherence tomography (OCT) in the early detection of AD is currently being studied. Our aim was to study the relationship between retinal nerve fiber layer (RNFL) thickness and AD. Methods We undertook an observational, analytical, cross‐sectional study with consecutive sampling of 32 patients with AD or mild cognitive impairment and a group of healthy controls (C). The total number of eyes studied was 64. An ophthalmological and a comprehensive neuropsychological evaluation were performed in all participants. Quantification of white matter lesions and study of atrophy of the hippocampus by cerebral magnetic resonance were also performed. Results We observed a significant linear trend towards a thinning of RNFL as the degree of cognitive deterioration increased, in the superior and temporal quadrants of the retina. A significant correlation was also noted between the mean thickness of the RNFL of the left temporal quadrant and occipital white matter lesions (r = −0.579, p = 0.038). Conclusions OCT could be a safe, rapid noninvasive tool providing useful biomarkers in the early detection of cognitive deterioration and AD.