A memory device may include a substrate; a first stack structure comprising a plurality of first gate layers and a plurality of first interlayer insulating layers alternately stacked on the substrate; a second stack structure comprising a plurality of second gate layers and a plurality of second interlayer insulating layers alternately stacked on the first stack structure; and a channel structure penetrating the first stack structure and the second stack structure, wherein the channel structure comprises a first portion in a first channel hole penetrating the first stack structure, a second portion in a second channel hole penetrating the second stack structure, and a first protrusion located in a first recess recessed into one layer of the plurality of first interlayer insulating layers from a side portion of the first channel hole.