A semiconductor device may include a first structure, a second structure on the first structure, and gate contact plugs penetrating through the first and second structures. The first structure may include a first stack structure including first gate layers and first insulating layers alternately stacked, a first pad capping pattern penetrating through at least a first portion of the first stack structure, and a first buffer capping pattern penetrating through at least a second portion of the first stack structure and spaced apart from the first pad capping pattern. The second structure may include a second stack structure including second gate layers and second insulating layers alternately stacked, and a second pad capping pattern penetrating through at least a portion of the second stack structure. The first gate layers may include first gate pads covered by the first pad capping pattern.