Provided is a three-dimensional semiconductor memory device include a first stack structure and a second stack structure adjacent to each other on a substrate, a first common source plug between the first stack structure and the second stack structure, a second common source plug between the first stack structure and the second stack structure, and a vertical dielectric structure between the first common source plug and the second common source plug. Each of the first stack structure and the second stack structure may include a plurality of insulation layers and a plurality of electrodes alternately stacked on the substrate. The first common source plug may be connected to the substrate. The second common source plug may be spaced apart from the substrate.