Three-dimensional (3D) semiconductor memory devices capable of improving reliability may be provided. For example, a three dimensional (3D) memory device, in which a plurality of memory cell strings are vertically arranged, may include a substrate, a stack structure of alternating a plurality of interlayer dielectric (ILD) layers and a plurality of gate electrodes, at least one of the ILD layers including pores, a vertical structure penetrating the stack structure and electrically connected to the substrate, and a data storage layer between the stack structure and the vertical structure.