Cognitive impairment is also an important cause of disability in MS in addition to motor, sensory, visual, and cerebellar affections. The aim of this study is to show the relation between the cognitive disability in MS with brain atrophy and retinal nerve fiber layer (RNFL). Forty-three multiple sclerosis (MS) patients, and 15 healthy individuals as controls were included in the study. MS patients were divided into three groups as relapsing-remitting MS (RRMS), relapsing-remitting with optic neuritis (RRMS+ON), and secondary-progressive MS (SPMS). An experienced psychologist performed modified Wechsler Memory Scale Revised form (WMS-R), Lines Orientation test, Stroop Color Word Interference test (STROOP), Standard Raven Progressive Matrices (SRPM), Benton Facial Recognition Test, verbal fluency test, and Paced Auditory Serial Addition tests in all cases. Optic coherence tomographies (OCT) were done. Cranial subcortical volumes of all subjects were measured using 3-dimensonal T1A imagines obtained by the cranial subcortical 1.5 tesla MR device (fully automatic Freesurfer program). Brain parenchymal fractions were calculated by proportioning the obtained volume measurements to the total intracranial volume. Fifty-eight subjects (65.5% female, 34.5% male) were enrolled in the study. There were significant differences among the groups in terms of parenchymal thickness, volumes of third ventricle, and white matter. There was a significant correlation between the volumes of the deep gray matter, mesial temporal structures and lateral ventricular volumes, and the test results of the WMS-R. OCT scores of all MS patients, whether or not they experienced optic neuritis, had increased, being worse especially in the SPMS group. Correlation between RNFL and the brain parenchymal fractions of the patients were statistically significant. Manual methods instead of automatic segmentation method are being more commonly used in the studies with brain atrophy and MS in our country. A significant correlation between OCT scores and brain atrophy is shown with our present study, and this is followed as a reflection of decrease in cognitive tests that provides valuable and reliable knowledge for the literature.