A semiconductor device includes a substrate including a memory cell region and a connection region, a plurality of gate electrodes stacked on the substrate, a channel structure penetrating the plurality of gate electrodes and including a channel layer extending in a vertical direction perpendicular to an upper surface of the substrate in the memory cell region, a dummy channel structure penetrating the plurality of gate electrodes and including a dummy channel layer extending in the vertical direction in the connection region, a first semiconductor layer disposed between the substrate and a lowermost one of the plurality of gate electrodes and surrounding the channel structure in the memory cell region, and an insulating separation structure disposed between the substrate and the lowermost one of the plurality of gate electrodes and surrounding the dummy channel layer.