Three-dimensional semiconductor memory device

A three-dimensional (3D) semiconductor memory device including: stack structures spaced apart from each other on a semiconductor substrate, wherein each of the stack structures includes interlayer insulating layers and semiconductor patterns alternately stacked on the semiconductor substrate; conductive patterns provided between the interlayer insulating layers vertically adjacent to each other and connected to the semiconductor patterns; and a protective structure covering a top surface of the semiconductor substrate between the stack structures, wherein a top surface of the protective structure is located between a top surface and a bottom surface of a lowermost interlayer insulating layer of the interlayer insulating layers.