A semiconductor device and a data storage system including the same, the semiconductor device including: a first structure including a peripheral circuit; and a second structure, including: a pattern structure; an upper insulating layer; a stack structure between the first structure and the pattern structure and including first and second stack portions spaced apart from each other, the first and second stack portions respectively including horizontal conductive layers and interlayer insulating layers alternately stacked; separation structures penetrating through the stack structure; memory vertical structures penetrating through the first stack portion; and a contact structure penetrating through the second stack portion, the pattern structure, and the upper insulating layer, wherein the contact structure includes a lower contact plug penetrating through at least the second stack portion and an upper contact plug contacting the lower contact plug and extending upwardly to penetrate through the pattern structure and the upper insulating layer.