A vertical semiconductor device includes a channel structure extending from a substrate in a first direction perpendicular to an upper surface of the substrate, and a ground selection line, word lines, and a string selection line sequentially formed on a side surface of the channel structure in the first direction to be separated from one another. The channel structure includes a protruding region formed in a side wall portion of the channel structure between the ground selection line and the upper surface of the substrate, the protruding region protruding in a horizontal direction perpendicular to the first direction.