Provided are three-dimensional semiconductor memory devices and electronic systems including the same. The device includes a substrate, stack structures each including interlayer dielectric layers and gate electrodes, which are alternately and repeatedly stacked on the substrate, vertical channel structures which penetrate the stack structures, and a separation structure, which extends in a first direction across between the stack structures. The separation structure includes first parts each having a pillar shape, which extend in a third direction perpendicular to a top surface of the substrate, and second parts, which extend between the interlayer dielectric layers from sidewalls of the first parts and which connect the first parts to each other in the first direction. The separation structure is spaced apart from the vertical channel structures in a second direction which intersects the first direction.