A three-dimensional semiconductor device is disclosed. The device may include first and second stacks separated from each other in a first direction, with each of the stacks including electrodes vertically stacked on a substrate. The device may also include vertical channel structures that penetrate the electrodes and are connected to the substrate, an interlayered insulating layer on top surfaces of the vertical channel structures, and a support pattern located between opposite sidewalls of the first and second stacks, in the interlayered insulating layer. A bottom surface of the support pattern may be positioned at a higher level than a top surface of an uppermost electrode of the electrodes.