Disclosed are three-dimensional semiconductor memory devices, methods of fabricating the same, and electronic systems including the same. The device includes a substrate including a cell array region and an extension region, stack structures extending in a first direction and including gate electrodes stacked on the substrate, vertical structures penetrating the stack structures on the cell array region, a mold structure on a portion of the extension region, a first support structure extending in the first direction between the stack structures, second support structures penetrating the stack structures on the extension region and spaced apart in a second direction from the first support structure, and a third support structure surrounding the mold structure in a plan view. Respective top surfaces of ones of the second support structures and a top surface of the third support structure is higher than a top surface of ones of the vertical structures.