A three-dimensional semiconductor memory device includes first and second gate stacked structures, disposed on a base substrate, and stacked in a direction perpendicular to a surface of the base plate, the first and second gate stacked structures including gate electrodes spaced apart from each other and stacked; a through region passing through the first and second gate stacked structures and surrounded by the first and second gate stacked structures; and vertical channel structures passing through the first and second gate stacked structures, wherein the first gate stacked structure has first contact pads adjacent to the through region and arranged in a stepped shape, the second gate stacked structure having second contact pads adjacent to the through region and arranged in a stepped shape, at least a portion of the second contact pads overlap the first contact pads on one side of the through region.