Objective Dementia frequently occurs in Parkinson’s disease (PD) but its pathophysiological basis is little known. Comparative EEG studies of Alzheimer’s disease (AD) and Parkinson’s disease dementia (PDD) are still rare, but could provide knowledge on the different pathophysiological mechanisms involved. The objective of the present study was to comparatively evaluate the absolute power and coherence on the EEG for patients with AD and PDD. Methods This study assessed 38 adults with AD, 12 with PDD, 31 with Parkinson’s disease without dementia, and 37 controls (CG) by a neurological evaluation, CERAD neuropsychological battery, executive functions tests and qEEG, calculating global absolute powers for the delta, theta, alpha and beta bands and inter- and intra-hemispheric coherences. Results The delta and theta powers were highest in PDD and lowest in CG (p <0.05). The beta frontal-occipital inter-hemispheric coherence was highest in PDD (p <0.05). Whereas, alpha and beta frontal inter-hemispheric coherence was highest in PDD and lowest in AD (p <0.05). Conclusion These results suggest that qEEG power and coherence measures are different in AD and PDD. Significance These qEEG differences must be related to the distinct mechanisms of cortical neural connections in AD and PDD.