Provided is a three-dimensional semiconductor device and method for fabricating the same. The device includes a first electrode structure and a second electrode structure stacked sequentially on a substrate. The first and second electrode structures include stacked first electrodes and stacked second electrodes, respectively. Each of the first and second electrodes includes a horizontal portion parallel with the substrate and an extension portion extending from the horizontal portion along a direction penetrating an upper surface of the substrate. Here, the substrate may be closer to top surfaces of the extension portions of the first electrodes than to the horizontal portion of at least one of the second electrodes.