Background: Cognitive decline in patients with Parkinson disease (PD) is a major and progressing health problem that needs reliable and objective assessment tools. Aim: Toexplore the value of EEG spectral ratio as cognitive biomarker in patients with PD. Methods: This cross-sectional case control study enrolled 35 patients with PD and 20 matched healthy controls. All participants were evaluated by quantitative electroencephalography (EEG) spectral power ratio (slow/fast) over different head regions, in addition to clinical and neuropsychological assessment of the patients using Unified Parkinson's Disease Rating Scale (UPDRS) and Montreal Cognitive Assessment (MoCA). Results: The UPDRS score of the patients was (mean 46.8 ± SD 26.6) and total MoCA score was (mean 20.3 ± SD 5.7). Twenty four of PD patients had cognitive impairment (MoCA <26) and showed significant higher spectral power ratio over the occipital region compared to PD patients with normal cognition (P=0.028). No significant differences of spectral power ratio between PD patients and controls. No significant correlation was found between power spectral ratio, UPDRS and MoCA scores. Conclusions: The occipital EEG spectral power ratio could be used as a complementary tool to neuropsychological assessment in evaluation and follow up of cognitive decline in patients with PD.