A three-dimensional semiconductor memory device includes a horizontal semiconductor layer provided on a lower insulating layer. The horizontal semiconductor layer includes a cell array region and a connection region. An electrode structure is provided including electrodes. The electrodes are stacked on the horizontal semiconductor layer. The electrodes have a staircase structure on the connection region. A plurality of first vertical structures are provided on the cell array region to penetrate the electrode structure. A plurality of second vertical structures are provided on the connection region to penetrate the electrode structure and the horizontal semiconductor layer. Bottom surfaces of the second vertical structures are positioned at a level lower than a bottom surface of the horizontal semiconductor layer.