According to example embodiments of inventive concepts, a three-dimensional semiconductor device may include: a memory cell array including memory cells that may be arranged three-dimensionally, the memory cell array including a left side opposite a right side, and a top side opposite a bottom side in a plan view; at least one word line decoder adjacent to at least one of the left and right sides of the memory cell array; a page buffer adjacent to the bottom side of the memory cell array; and a string selection line decoder adjacent to one of the top and bottom sides of the memory cell array.