A semiconductor device capable of improving operation performance and reliability, may include a gate insulating support to isolate gate electrodes that are adjacent in a length direction. The semiconductor device includes a first gate structure on a substrate, the first gate structure extending lengthwise in a first direction to have two long sides and two short sides, relative to each other, and including a first gate spacer; a second gate structure on the substrate, the second gate structure extending lengthwise in the first direction to have two long sides and two short sides, relative to each other, and including a second gate spacer, wherein a first short side of the second gate structure faces a first short side of the first gate structure; and a gate insulating support disposed between the first short side of the first gate structure and the first short side of the second gate structure and extending lengthwise in a second direction different from the first direction, a length of the gate insulating support in the second direction being greater than a width of each of the first gate structure and the second gate structure in the second direction.