A three-dimensional semiconductor device includes gate electrodes including pad regions sequentially lowered by a first step portion in a first direction and sequentially lowered by a second step portion in a second direction perpendicular to the first direction, the second step portion being lower than the first step portion, wherein a length of a single pad region among pad regions sequentially lowered by the second step portion in the second direction is less than a length of a remainder of the pad regions in the second direction.