A three-dimensional semiconductor device and a method of fabricating the same, the device including a lower insulating layer on a top surface of a substrate; an electrode structure sequentially stacked on the lower insulating layer, the electrode structure including conductive patterns; a semiconductor pattern penetrating the electrode structure and the lower insulating layer and being connected to the substrate; and a vertical insulating layer interposed between the semiconductor pattern and the electrode structure, the vertical insulating layer crossing the conductive patterns in a vertical direction and being in contact with a top surface of the lower insulating layer.