A semiconductor device includes a pattern structure; a stack structure including gate layers stacked in a first region on the pattern structure and extending into a second region; a memory vertical structure penetrating the stack structure in the first region; gate contact plugs electrically connected to the gate layers in the second region; and a first peripheral contact plug spaced apart from the gate layers, the gate layers including a first gate layer, the gate contact plugs including a first gate contact plug electrically connected to the first gate layer, side surfaces of the first gate contact plug and the first peripheral contact plug having different numbers of upper bending portions, and the number of upper bending portions of the side surface of the first gate contact plug being greater than the number of upper bending portions of the side surface of the first peripheral contact plug.