A semiconductor device includes gate electrodes and interlayer insulating layers that are alternately stacked on a substrate, channel structures spaced apart from each other in a first direction and extending vertically through the gate electrodes and the interlayer insulating layers to the substrate, and a first separation region extending vertically through the gate electrodes and the interlayer insulating layers. Each gate electrode includes a first conductive layer and a second conductive layer, the first conductive layer disposed between the second conductive layer and each of two adjacent interlayer insulating layers. In a first region, between an outermost channel structure and the first separation region, of each gate electrode, the first conductive layer has a decreasing thickness toward the first separation region and the second conductive layer has an increasing thickness toward the first separation region.