Background: Parkinson’s disease (PD) is a progressive neurodegenerative disease of aging. Optical coherence tomography (OCT) is a simple noninvasive imaging technique to estimate the retinal nerve fiber layer (RNFL) thickness. The aim of this study is to measure the changes of the RNFL thickness in patients with Parkinson disease in comparison with the control group and to detect the relationship between these changes in RNFL with cognitive functions and physical disability in Parkinson patients. Methods: 20 PD patients (40 eyes) and 20 healthy persons (40 eyes) were enrolled according to the defined criteria. The diagnosis of the patient's Parkinsonism was done according to the brain bank clinical diagnostic criteria for idiopathic Parkinsonism and the severity of the disease was assessed using the Hoehn and Yahr scale. The Montreal Cognitive Assessment (MoCA) was used for cognitive evaluation in all participant subjects. Quality of life was evaluated by Parkinson's disease questionnaire-39 (PDQ-39). Optical coherence tomography (OCT) was done for all patients and control subjects. Results: The mean RNFL thickness was significantly reduced in Parkinsonian patients with cognitive impairment (73.64±1.81 μm) when compared with those without cognitive impairment (77.25±4.53 μm) and control subjects (84.1±6.98 μm) (P < 0.001). The retinal thickness reduction was statistically significant in superior and inferior quadrants of the retina (P < 0.001, P =0.03 respectively).The thickness of the superior RNLF correlated significantly with both the Montreal Cognitive Assessment (MoCA) (r = 0.59; P < 0.001) and the mobility scores according to PDQ 39 scale (r= -0.39; P=0.012). Conclusion: Parkinson disease is associated with distinct retinal pathological changes. OCT seems to be a sensitive tool for detecting these retinal changes and correlated significantly with the progression and severity of Parkinson disease. Keywords: Parkinson, Retina, Retinal nerve fiber layer, Optical coherence tomography, cognitive dysfunction