A three-dimensional semiconductor memory device is provided. The memory device includes a substrate with a cell array region and a connection region adjacent to the cell array region, the connection region including a first pad region and a second pad region; an electrode structure including electrodes stacked on the substrate, the electrode structure including an upper portion forming an upper staircase structure; a first dummy structure laterally spaced apart from the upper portion of the electrode structure and provided on the first pad region; and a second dummy structure laterally spaced apart from the upper portion of the electrode structure and provided on the second pad region. Each of the first dummy structure and the second dummy structure includes a dummy staircase structure, and the first dummy structure is located at higher level than the second dummy structure.