A three-dimensional semiconductor memory device including a substrate including a first connection region, a second connection region, and a cell array region disposed between the first and second connection regions. The memory device further includes an electrode structure including a plurality of electrodes vertically stacked on the substrate, wherein each of the electrodes has a pad exposed on the first connection region, and a dummy electrode structure disposed adjacent to the electrode structure and including a plurality of dummy electrodes vertically stacked on the substrate. Each dummy electrode has a dummy pad exposed on the second connection region. The electrode structure includes a first stair structure and a second stair structure which each includes the pads of the electrodes exposed on the first connection region. The first stair structure extends along a first direction, and the second stair structure extends along a second direction that crosses the first direction.