Disclosed is a three-dimensional semiconductor memory device that includes first to third channel groups arranged in a first direction on a substrate. The first to third channel groups are spaced apart from each other along a second direction on the substrate. Each of the first to third channel groups includes a plurality of vertical channels that extend in a third direction perpendicular to a top surface of the substrate. The first and second channel groups are adjacent to each other in the second direction and spaced apart at a first distance in the second direction. The second and third channel groups are adjacent to each other in the second direction and are spaced apart at a second distance that is less than the first distance.