A semiconductor device includes a substrate, a stack, and channel structures penetrating the stack. The stack includes gate electrodes and insulating layers alternately and repeatedly stacked on the substrate, and extending in a first direction. The channel structures in a first row are spaced apart from each other in the first direction. The stack includes a first sidewall that includes first recessed portions and first protruding portions. Each of first recessed portions is defined by an adjacent pair of the first recessed portions. Each of the first recessed portions has a shape recessed toward a first region of the stack between an adjacent pair of the channel structures of the first row. Each of the first recessed portions has a width that decreases in a direction toward the first region when measured along the first direction.