A three-dimensional semiconductor device includes gate electrodes sequentially stacked on a substrate, a channel structure penetrating the gate electrodes and being connected to the substrate, an insulating gap-fill pattern provided within the channel structure and surrounded by the channel structure as viewed in a plan view, and a conductive pattern on the insulating gap-fill pattern. At least a portion of the insulating gap-fill pattern is received in the conductive pattern, and at least a portion of the conductive pattern is interposed between at least that portion of the insulating gap-fill pattern and the channel structure.