Disclosed are three-dimensional semiconductor memory devices, electronic systems including the same, and methods of fabricating the same. The three-dimensional semiconductor memory device includes a substrate including a cell array region and an extension region, a peripheral circuit structure including peripheral transistors on the substrate, a stack structure including interlayer dielectric layers and gate electrodes that are alternately stacked on the peripheral circuit structure, contacts that penetrate the stack structure on the extension region and are electrically connected with the peripheral transistors and include a protruding part contacting a sidewall of one of the gate electrodes and a vertical part penetrating the stack structure, and dielectric patterns between the vertical part and respective sidewalls of the gate electrodes. Top and bottom surfaces of each of the dielectric patterns are respectively in contact with adjacent ones of the interlayer dielectric layers.