A three-dimensional semiconductor device includes an electrode structure on a substrate that includes a first region and a second region, the electrode structure including a ground selection electrode, cell electrodes, and a string selection electrode which are sequentially stacked on the substrate wherein the ground selection electrode, the cell electrodes, and the string selection electrode respectively include a ground selection pad, cell pads, and a string selection pad which define a stepped structure in the second region of the substrate, a plurality of dummy pillars penetrating each of the cell pads and a portion of the electrode structure under each of the cell pads, and a cell contact plug electrically connected to each of the cell pads, wherein each of the dummy pillars penetrates a boundary between adjacent cell pads, and wherein the adjacent cell pads share the dummy pillars.