A semiconductor memory device includes; a first stacked structure including a first staircase portion, a second stacked structure on the first stacked structure and including a second staircase portion overlapping the first staircase portion, a first contact plug penetrating the first stacked structure and the second stacked structure, electrically connected to the first stacked structure and not electrically connected to the second stacked structure, and a second contact plug penetrating the first stacked structure and the second stacked structure, electrically connected to the second stacked structure and not electrically connected to the first stacked structure.